What did you search for on Google? I typed in "n900 android" and it gave me quite a few relevant pages mentioning something called "Nitdroid"...

Hmmmm, I thought. I wonder what this "Nitdroid" thing is? So I typed that into Google. Ohhhh look at that - there's a website called "www.nitdroid.com". Must be worth a look, I thought. On the first page you come to it says: "This project is hoping to create a stable and working port of the Android OS for the Nokia Internet Tablets (N8x0 and N900). "

Da daaaaaa! That's how you do it.

Oh, I don't think you've found that you've actually got a fully functional qwerty keyboard - slide it open. (There's a picture in the User guide - page 13)

--edit--

Also:
http://wiki.maemo.org/
Type 'android' in search box.
Select 3rd link ("Alternative Operating Systems").
Click "9. Android".
Cick "NITDroid".