Spammers are always going to adapt to new systems.
Now then, as to your suggestions:
CAPTCHA: It's not accessible to blind users and would require us to either use PHP sessions (which we use nowhere else) or add our own system for storing data in, say, the online table, both of which are less than ideal. Of course, people are always looking to crack CAPTCHAs using OCR software, so we would probably end up in another arms race against spammers.
Random email: I'd be interested to see how you would do that, I don't think it's actually possible. Of course, each individual forum can change its welcome email to be unique, which is a point I'll be addressing right now
Now then, for 1.3, we plan to release several extensions to help deal with spam. The key to addressing spam, however, is realizing how much of it is automated. Just adding a trick to every forum won't help, since spammers will code their bots around it. So, we need to focus on allowing each forum to make its setup unique.