I like this plastic holster from AT&T:
link. I find it to be VERY fast to get in and out. I use it as a horizontal holster but it can be vertical too. Very low profile compared to the original case. It locks very tightly into the slots on the top of the BB and it has the magnetic sensor. You should be able to pick it up at an AT&T store if you have one close.
For my Curve I had the Seidio case that was somewhat similar. It was a rubbery coated plastic with felt on the inside, but it used a big spring clip that came all the way around the device. After a couple of months that clip damaged the top of my Curve. I find this simple plastic one to be far superior and does not appear to damage the device at all. You can take it out and put it in with one hand very easily.