Akismet works well for me. It caught over 100,000 spam messages on my sites last year and I only found 2 or 3 that were snagged incorrectly.
When you find comment spam that got past Akismet, it is important to click on the spam link to identify it as spam. Do not delete it. When you click the spam link it gets reported to Akismet, When they see enough spam submissions for that message, it gets added to their spam message profiles.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin
I think the solution is just to remove the url from the comments altogether. On my main site I approve around 100 comments per day - none of them with a url. I do get a spam comment ever so often but no more than 3 or 4 in a weeks time. People comment on my blog because they are interested in the content and not because they are trying to get some sort of backlink.
I do have a few other blogs where I allow a url and I have been noticing both google.com and yahoo.com in the url field lately. Oh and on most of my other blogs I have also removed the url field and will probably do the same to the rest of them.
I have not seen this thing. But in some of the comments i have seen the link of the same blog where the comment is being posted. Is this a spam as well?