I've been thinking about this quite a bit. Google's proposed gmail will 'read' your email and based on words/subjects/etc that it finds it will target ads at you whenever you go to the site to read your mail. It will store up to one gigabyte of email for you and use all this 'history' to datamine subject matters that might be interesting to you. On the surface (if you ignore the glaring privacy concerns) this might seem like a good idea. Why waste time showing me adds for golf resorts when I don't golf? But show me an add for a tropical get-away at a dive resort and I might just click!
Of course, as it turns out this is a really bad idea. Not that I wouldn't mind getting ads showing me luscious beach-scapes and crystal clear oceans. It's just that very little of my mail actually pertains to anything I'm at all interested in. What's this? Don't believe me? The truth is, 90 percent of all mail messages I recieve on my 'public' accounts are spam, messages sent out by disreputable personages trying to hock their wares. So, if google uses this data to determine what I'm interested in, then all they'll find are subjects pertaining to insurance, home loans, pornography, viagra and various other unmentionables.
I really don't need more pop-up ads targetting this stuff.