If you are looking for A2DP, you may have to get one with 3G on the side because that seems to be the trend for manufacturers, sticking all the "cutting edge" tech in the same devices, especially if you are going though Cingular to get your phone.
If you are willing to go unlocked there's a quite a few options. The most comprehensive list I've seen of A2DP compatible devices can be found below:
http://www.robertwebbe.nl/A2DP.html
Personally I'm going for the Sony Ericsson K790a, it has a decent camera and although it isn't walkman branded, you can use the wired headphones similar to the other Walkman phones and of course it has A2DP if you want to go wireless.
Samsung also has a bunch of decent phones with A2DP. I'd give the D900 a look, it's a thin slider w/ decent camera as well.
If you want to wait a while, Moto is coming out with a RAZR slider, a narrow type razr and a couple of 3G RAZRS all with A2DP.
Oh the ROKR e2 is already out and I think the Ming A1200 also has A2DP, but I'll have to double-check.
