There is an interesting bit of news floating around that says Google does not index webpages whose URL ends in a zero. This particular type of address typically signals spam and therefore Google does not think highly of these types of pages.

Example of a page with the type of URL in question:
en.wikipedia.org/wiki/Windows_1.0

For more information, take a look at the article posted on SEOmoz.

As it turns out, this filter appears to be limited to URLs that end in zero. Other numbers have been getting indexed OK.