Have you any information on low-noise muffler technology--especially
mufflers that could be purchased and retrofitted on existing, in
service commericial lawnmowers?  Thanks.

Hello dargan, here is some info I found that answers your question:

First for new muffler technology:

http://researchnews.osu.edu/archive/fibrmufl.htm

COLUMBUS, Ohio A study of muffler technology at Ohio State University
is giving American automakers new options for designing quieter cars.

Fiber-filled mufflers have been used in European and Japanese cars for
years, Selamet explained, but not much elsewhere. In North America,
most mufflers work by using metal chambers and baffles to slow the
flow of air or redirect it.

But chambers and baffles can restrict the flow of the exhaust gases,
increasing what is known as back pressure. When that happens, some of
an engines work is wasted pushing the burned gases through the exhaust
system, instead of pushing the car forward. With a simpler interior
design, a fiber-filled muffler could cause less back pressure and make
engines more efficient.

In tests, Selamet and his colleagues found that the fiber reduced
engine noise substantially. For example, at the mid-range frequency of
1500 Hertz, the new design reduced the noise by 40 decibels. Thats
significantly higher than the typical muffler rating of 30 decibels or
lower.

http://www.thevettedoctors.com/corsa_rsc.htm

Automobile makers have been building mufflers for over 100 years, and
as the name muffler implies, the primary purpose of the muffler is to
reduce the noise emitted by the internal combustion engine. Muffler
technology hasn?t changed very much over the past 100 years. The
exhaust is passed through a series of various-sized chambers in
reactive-type mufflers, or straight through a perforated pipe wrapped
with sound deadening material in an absorption-type muffler. Both of
these types of mufflers have their strengths and weaknesses. The
reactive-type of muffler is rather restrictive and prevents even the
"good" engine sounds from coming through but does a good job of
reducing noise. On the other hand, most absorption-type mufflers are
less restrictive but allow too much engine noise to come through,
especially at cruising speeds. And regardless of the packing material,
absorption-type mufflers tend to get noisier with age.

More recently automotive engineers have been experimenting with
electronic noise suppression mufflers. A sound pressure wave
180-degrees out of phase is generated by an electronic device to
cancel out a similar sound wave generated by the engine. It is an
effective way of canceling noise without restricting flow.
Unfortunately it is too costly and currently impractical for todays
production vehicles.

CORSA Performance has developed a patent-pending method of reflecting
sound pressure waves within the muffler case to produce the same
180-degree out-of-phase cancellation wave effect, with no flow
restriction and no electronic devices. It literally took the
proverbial "rocket scientist" to come up with the design. An
aeronautical engineer with years of fluid flow and gas dynamics,
thermodynamics and acoustics experience on NASA and numerous other
government projects was employed to adapt some of this space-age
noise-suppression technology to automotive mufflers. The result is the
CORSA Power-Pulse? Reflective Sound-Cancellation Technology, or RSC
for short.

Next lawn equipment mufflers:

http://www.cleanerfuture.com/products/quietcatproduct.htm

The Quiet Cat? Catalyst/muffler system is designed to be a direct
bolt-on application for the small utility engine sector of our
business. This market includes industrial construction equipment such
as mixers, concrete floats, small generators and water pumps to garden
equipment such as lawn tractors, rotor-tillers, trimmers, mowers and
leaf blowers.
The Quiet Cat? uses ESW's advanced woven steel mesh catalytic
substrate technology at the core of its emission and noise reduction
processes. The ability to mold the substrate into a myriad of shapes
and sizes allows for the product to be fitted into even the tightest
of spaces without forgoing any of its performance properties.
All Quiet Cat? Catalyst/muffler systems have been engineered in the
company's Telford P.A. based R&D facility. Each unit is specifically
designed and thoroughly tested for each engine application.
Environmental Solutions Worldwide is fully ISO 9001: 2000 compliant.
This means that all Quiet Cat? catalyst/muffler systems are
manufactured to the highest quality standards in our Telford PA
facility. Every Quiet Cat? catalyst/muffler system passes through
stringent quality control checks before being serialized, boxed and
shipped to the customer.

http://www.jackssmallengines.com/Quietmuf_index.html

Jacks lists quiet mufflers to quiet the exhaust of large and small
engines. Three types are available. Super TrappTM Mufflers are made to
retrofit almost any engine. Also, there are quiet mufflers available
for selected Tecumseh and Briggs and Stratton engines where the Super
TrappTM muffler will not work. Look Below!

Super TrappTM Quiet Mufflers 
Will retro fit to most engines and can be made quieter with a larger
resonator. This muffler is our favorite.
http://www.jackssmallengines.com/strapmain.cfm

Briggs & Stratton Quiet Mufflers 
Available for Briggs and Stratton Engine Models 190000-19ZZZZ, 220000
and 250000 powered Generators and Lawn Equipment.
http://www.jackssmallengines.com/bQuietmuf_index.html

Tecumseh Quiet Mufflers 
Available for Tecumseh Engine Models HM 70-80-100, TVM Models and
TVXL-170,195,200 powered Generators and Lawn Equipment.
http://www.jackssmallengines.com/bQuietmuf_index.html

This should take care of your commercial mower quiet muffler requirements.

Please let me know if you need any further info, Redhoss

This guy is so good it's scary.  1.  technical overview.  2. Specific
instance, as requested, answering original question pretty cloesly. 
3.   Future prospects.  Who is this, the moonlighting editor of
Popular Mechanics?