Today, September 27, 2009 is Google's 11th Birthday.
Get it? Googlle? If you hover over the Google Doodle, you get a hint to what they're on about.
Its written that way as a typographical nod to their anniversary. When we used to use manual typewriters, we used to have one font for the most part, Courier. Many typewriters, including the Post War Era beast of a manual that I grew up using for term papers and the like simply did not have a 1 key. A discrete key for the digit One was overkill because a lower case l (L or "Ell") looked like a one and people just typed that way. The whole Leet Speek is an echo of that where they use numbers to represent letters in an effort to confuse those outside of their own clan. Spelling Elite as 1337 and the like.
Personally I prefer Gill Sans and use it everywhere, but people sometimes wonder why my printed correspondence looks like it was written by London Underground.