SBL84807 tells the story: Spamhaus has observed Twitter invitations more-or-less being used by spammers. According to Spamhaus, Twitter does not appear to have controls in place that prevent spammers from issuing invitations to imported lists of email addresses, and also, Twitter invitations have a broken unsubscribe link.
Let's hope Twitter works quickly to address this issue to Spamhaus's satisfaction.
I personally am not a big fan of "import your address book and we'll send everybody you've ever talked to an invitation to our fabulous new social network," as address books are invariably filled with crap. Even if the intent isn't nefarious, if I did this, I'd end up sending invites to the Apple store, all the mailing lists I'm on, the various abuse desks I talk to, including Twitter's own Del Harvey.
Also, people seem way to willing to hand their email passwords over to third parties. I'm sure Twitter isn't planning on stealing your address book, but what of the next site? And the site after that? Eventually a bad guy will figure out that this is a great way to harvest your contacts.