I've only got one prediction for 2008, and it's this: Spam is going to be even less tolerated by internet service providers than it is in 2007.
ISPs are continually tightening up their sending guidelines and acceptable use policies, and things you might have gotten away with in 2006 or 2007 will no longer be kosher.
Opt-out append? Purchased lists? Third-party lists? Mailing to the same, tired list forever? Forget about it. You're going to the bulk folder, if you get through at all.
ISPs are belt-tightening; automating sender-review and spam-prevention processes. Spam isn't a profit center for them; it sucks up their resources that they feel are better spent elsewhere. They're taking less and less time to individually review every whitelist request; they're relying more on automated, statistics-driven processes to keep more of the spam out, and they're catching more and more edge case senders in their new mechanisms. ISPs aren't making any money from the mail you're sending, they don't have a financial responsibility to accept that mail. And in a lot of cases, they firmly believe that their users are happier without the mail
It's up to you if you want to stay ahead of this problem, and stay in the inbox. The way to do it is avoid becoming that edge case. Maintain clear permission. Don't buy or sell lists. Avoid email append. Re-confirm your lists. Send people only what they expect.