Filter spam URLs

[Ned Rockson]

I've been searching for a bit on the forums to see if anyone is in the 
process of producing a spam filter heuristic for URLs.  I assume that 
most spam is nondeterministic, but after a crawl of ~50M URLs, there are 
a bunch that are obviously spam because their URLs are simply 
nonsensical (like 01118273.domain.com I would automatically filter 
out).  Is anyone currently working on this or has there been any effort 
in the past?  Also, does anyone know of any literature published about 
this?  A quick google search netted only email spam filters using naive 
bayes.