From what I've heard, it'll be $200 cheaper than the current gen iPhones, available as 8 and 16 GB models, slightly thinner, 3G-capable, will support MS Exchange, and will support CUT & PASTE (something I've been bitching about since I got mine last June!).
Other than that, it's got the best browsing capability I've ever used in a smart phone, lags behind slightly in email support compared to the Blackberry (no push service, although it does support POP and IMAP), and about even with regards to tasks, schedules, contacts, etc. And it's a wifi-capable video iPod.
Sometimes I love mine, other times I want to smash it against a brick wall.
EDIT: If you get a lot of voice mail messages, visual voice mail is the best. Such a simple idea I wonder why it was never introduced prior to the iPhone.