PS.... If you want the ultimate.... You can already get wireless 2-way synchronization on all folders between an Exchange-powered IMAP server, but you must use Blackberry Enterprise Server to gain this 2-way wireless email synchronization to BlackBerry.
It's even more expensive than just having a BlackBerry plan -- you also have to pay for Blackberry Enterprise Server on top of it all! But this is the Mercedes of wireless email.
If you want wireless bidirectional email synchronization, you're limited to few choices of servers, basically Exchange is your only choice if you want the most inexpensive $10-to-$25-per-month wireless email synchronization (Exchange/BES/MDS hosting by Mailstreet or MyBlackberry, etc). Not many people like Exchange, because it's proprietary though.
(Granted, you can use Goodlink on a TREO 600, but that's more expensive for a single user, unfortunately)