Let us see
The T637 is the direct upgrade, but doesn't offer much more (nicer screen, a few other things).
The SLVR V8 might be able to use bluetooth iSync and it's out in Q3. Think candybar RAZR V3. However it might not, and it will probably be fairly expensive. Pics:
SLVR V8 compared to RAZR V3
Also the PEBL V6, but that looks like an end of the year phone. And again, it might not be able to bluetooth iSync.
There is the Sony Ericsson K700.
Review here
The K750 is an improved model nicknamed
"Clara" that should come out some time later this year.
There's the K600
first look here however it is a 3G phone and as such many of its features are useless in North America for some time.
As mentioned there is also the Z800 (i.e. a rebranded V800) but I don't believe it supports the 850 band (needed for Cingular/Rogers)
Finally there's SE's S710a which is the only phone I've listed that will be out in the next few weeks (with Cingular) but it is rather big,
check out a review here, and it is a swivel which some people don't like.
I've concentrated on SE phones simply because SE phones work great with Macs (see Sailing Clicker) and have arguably the best menus. I'm not a huge fan of Nokia anymore so I don't really follow their phones.
I've included the Motorola's because I really like their current industrial design efforts on the high end, re: RAZR V3, PEBL V6, and the SLVR V8. However I don't know whether or not they have changed the UI and added bluetooth iSync support in the V6 and V8.
Frankly the T610/616 held up fairly well with one upgrade (T637) and a kind of successor in the K700.
Release dates are generally international as US Cellphone carriers take quite some time to get phones.
I've used Engadget because they're decent, and have a search function - I didn't feel like googling.
Additionally all of these are GSM/GPRS/EDGE or 3G UTMS (i.e. video phones, fast web browsing and the like) phones and as such are offered by a limited selection of carriers in the US. Especially since 3G will not be fully rolled out for some time in North America.
To note, a number of these phones are triband 900/1800/1900 and as such are less usable with Cingular/Rogers in NA. Some of them will probably be released in Triband 850/1800/1900 models like the S710a (originally the S700i).
Hope some of that helps. I can go more in depth if a couple catch your interest.