I don't know what the deal with Yahoo! Mail is lately but the increasing levels of spam is troubling. Ever since the switch to an AJAX-powered model, spam has been simply out of control. Much of the spam are porn-related and dealings with prescription pharmaceuticals (the usual suspects).
I would gladly switch to another e-mail provider, but switching e-mail addresses these days is akin to switching phone numbers-- a royal pain in the ass; something to be avoided at all costs. Yahoo! Mail is my lifeline in this electronic world, so I'm willing to to tolerate a certain level of spam. Nevertheless, I have my limits, and unless Yahoo! rectifies the situation quickly, I will be forced to go elsewhere, like GMail, whose spam filter works much better than Yahoo's.