The current bitcoinpayment system is pretty lightly engineered - just adding people to a group when they send a payment. (It was made by MagicalTux, for the record).

Requiring payment even before creating a user would take some thinking and coding, and as you can guess, Mtux is pretty swamped with mtgox stuff right now, so don't hold your breath. :)

We've also talked about setting up an automatic sweeper bot to wipe new accounts that never verified, but again - time constraint. Mtux runs the wiki, so he's the one who'd have to do it.

In the meantime, the spam user creation problem can be taken care of by just filtering out the User namespace from the recent changes list. It's not perfect, since it also kills edits of user pages, but it really does go a long way. Try it:
https://en.bitcoin.it/w/index.php?namespace=2&invert=1&title=Special%3ARecentChanges