ryano has it right. You crazy Americans have twisted the use of the word LEGO, assuming that it works in a plural form. Not so.

See LEGO's fair use page, http://www.lego.com/info/fair.asp, which describes how the LEGO trademark may be used...

If the LEGO trademark is used at all, it should always be used as an adjective, not as a noun. For example, say "MODELS BUILT OF LEGO BRICKS". Never say "MODELS BUILT OF LEGOs".
Ahha! Further evidence is to be found in the rec.toys.lego FAQ
ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/rec/toys/lego/LEGO_frequently_asked_questions_%28FAQ%29
One catalogue, dated 1980, has the following on its back page: Dear Parents and Children
The word LEGO(R) is a brand name and is very special to all of us in the LEGO Group Companies. We would sincerely like your help in keeping it special. Please always refer to our bricks as 'LEGO Bricks or Toys' and not 'LEGOS.' By doing so, you will be helping to protect and preserve a brand of which we are very proud and that stands for quality the world over. Thank you!

Susan Williams
Consumer Services

So, there you go. The definitive answer to the LEGO / Legos debate.