Email addresses are worthless for much verification nowadays. I use SpamBob.net for most e-mail addresses I give out now, but there's nothing stopping me giving out a .com address to really shady sites.
I figure maybe, the best way to let people register, is not make it obvious the function exists. A user posts as a guest under a nickname, but maybe an optional form field, like, ''Registration brings certain benefits to posting. If you want to register, enter your email address in this field. You can use the SpamBob.net service if you want to remain anonymous.''
Then it could make their post, but remember that they wanted to register. It could send an email to the address providing a link to register (something like register.php?post=123&key=456789) and when they completed registration, it would convert the post they made to a full one.