What is the penetration of the .NET platform on business desktops in the US?
In other words, how many business computers are out there with the
20MB runtime which users must install to run .NET based software (such
as software written in C#)?
Here are some relevant, or semi relevant links I found:
http://www.itwriting.com/blog/?postid=64
http://www.assetmetrix.com/forms/index.asp?template_id=13&intNew=79
(this one is more related to the adoption of XP, but can provide a
hint)

To be clear - what I am looking for is the number of "professionals"
or business people (as opposed to pure consumers) who will need to
download the ~20MB .NET framework in order to run a .NET based
application on Windows.

I need some validation for the answer ? so please provide all the sources you used.

Thank you!

According to this source (a blog by a visual c# manager at Microsoft
who says this is from Microsoft's marketing department)

    *  More than 120M copies of the .NET Framework have been
downloaded and installed using either Microsoft downloads or Windows
Update
    * More than 85% of new consumer PCs sold in 2004 had the .NET
Framework installed
    * More than 58% of business PCs have the .NET Framework
preinstalled or preloaded
    * Every new HP consumer imaging device (printer/scanner/camera)
will install the .NET Framework if it?s not already there ? that?s 3M
units per year
    * Every new Microsoft IntelliPoint mouse software CD ships with
the .NET Framework

http://blogs.msdn.com/scottwil/archive/2005/03/09/391199.aspx

This is in the direction, thank you! 
Note though:
1. I am looking at business (incl. SOHO) computers
2. Info from Microsoft is good, but may be tainted - I will need
validation from unaffiliated sources

Unfortunately, that's all the info I can find.  You question is
difficult because of similar issues mentioned in that blog.

That is how many companies wipe the machines they receive and then
reinstall with an image that contains or does not contain .NET?

I can tell you that all new PCs that our organization purchases come
with the .NET framework pre-installed.

If you are planning to develop software that will be shipped you may
consider just bundling in the .NET framework with your software
package.

http://msdn.microsoft.com/netframework/downloads/framework1_1/

The penetration of .NET will roughly parallel the statistics for
applications that require it in their install, such as Quickbooks,
plus some fudging for includes.  There likely isn't an accurate way of
estimating this, at least on that kind of budget.  I could give you a
short answer, but it would likely arrive with a tall suit.