One of the challenges of buying a new Windows Phone is finding accessories for it. Most notably, finding a case that fits. Whether you've purchased the new HTC HD7, Samsung Focus or the HTC Surround more than likely the case for your older Windows Phone is going to be too small. One case style that seems to work the best are those fitted for the HTC HD2.
I choose the Samsung Focus for my Windows Phone and have been using the SPE Top Case for the HD2. The Focus fits nicely in the case with just enough wiggle room to make placement and removal easy. The case itself is well built and I like the brown leather.
SPE makes another case that works well with the HTC Surroud, the SPE Side Case for the HD2. The Surround fit in the side case nicely and the case was comfortable to wear. While the Focus fit the Side Case, due to the thinner form factor, the Focus bounced around alot in the case.
The new Dickey Side Case for the HD2 worked well but some may not like the Velcro closure or belt loop (as opposed to a belt clip). Then there's always the Golla Cases that have plenty of room to carry your Windows Phone, a few credit cards and cash.
It's been suggested in a forums discussion that cases for the Samsung Captivate also works well with the Focus. If you've found a case that fits just right with any of the new Windows Phones, share it in the comments or join in on one of the WPCentral Forums discussions on accessories.