Lexan is a GE brand name, but the most commonly used word, when
referring to polycarbonate sheets.  It seems that most of the sheet
sellers use polycarbonate and lexan interchangably, when the might
only be selling another brand name of polycarbonate.

Kind of like "Formica" and "plastic laminate".
At what point does a brand name become so commonly used that it is OK to 
publish a website with that brand as part of the title, even though it
is explained in more detail on the page, like:
"lexan equivalent"

Trademarks can become generic if they become commonly used for a type
of product or service.  If a trademark becomes generic, the owner may
no longer be able to legally enforce their rights to the trademark. 
Trademark owners can reduce the risk of this occurring if they make
efforts to avoid using the trademark in a generic manner and
consistently enforce their trademark rights.

So, in the case of the web sites using the term Lexan® generically,
they are either using it improperly, or GE Plastics is running the
risk of the trademark becoming generic if the company is not actively
enforcing its trademark rights by asking them to stop.  Given that GE
Plastics makes every effort on its web site to use the Lexan®
trademark properly, including using the registered trademark symbol
and using the phrases like, "CYCOLAC, CYCOLOY, GELOY, LEXAN, NORYL,
STARFLAM, SUPEC, THERMOCOMP, ULTEM, VALOX, VERTON, Visualfx and XENOY
are registered trademarks of General Electric Company," in its
publications and on its web site, it appears that the company is
trying to avoid the trademark becoming generic.  As a result, the web
sites are likely to be infringing upon it and may be the subject of a
trademark enforcement action by GE Plastics.  Their usage of the term
to advertise competing products would not appear to fall under proper
"fair use" of the trademark.

I have included a number of articles below describing issues related
to trademarks and how they can become generic.

Sincerely,

Wonko

Sources:

"Lexan® PC Resin Product Brochure" GE Plastics
http://kbam.geampod.com/KBAM/Reflection/Assets/6217_5.pdf

"Genericized trademark" Wikipedia (June 6, 2006)
http://en.wikipedia.org/wiki/Genericized_trademark

"Trademark" Wikipedia (June 6, 2006) http://en.wikipedia.org/wiki/Trademark

"Strength of Trademarks" by Daniel A. Tysver (2005)
http://www.bitlaw.com/trademark/degrees.html

"Fair Use of Trademarks" by Lloyd L. Rich (2002) The Publishing Law
Center http://www.publaw.com/fairusetrade.html

"Using the Federal Registration symbol (the letter R enclosed within a
circle) "®" with a mark" MyCorporation.com (2006)
http://www.mycorporation.com/trademark/symbols.htm

Search terms: trademark generic; registered trademark symbol; GE Lexan trademark

Very complete, not sugar coated.

This subject has been discussed in an earlier question, but I won't
deprive a Researcher of the pleasure of answering.  There is a word
that describes a trademarked name becoming generic.