I think it really depends on the BB that you have (obviously). When I had my curve, I had a silicone case and it stayed in my pocket.
But now that I have a 9000, I found that it had a hard time coming out of my pocket with a silicone case on. So I now rock a naked 9000 which lives 50% in my pocket, 50% in my hand. Usually when I sit down I have it on the table in front of me so I don't have to squeeze the monster out of my tiny pockets...
I've got a screen protector and then an Epik case on my 8530. It usually goes in either my front or back pocket, depending on whether or not my jeans have buttons on the back pockets or not. (Buttons=front pocket carriage. Otherwise I'll sit down with it and managed to crack the screen. :/)
Inside the Leather pouch, in my front pocket.
I am not a fan of belt holsters... This is small enough to go on my pocket.
I carry my BB 9000 in the belt/swivel holster that came with it because it has the proximity feature that shuts off the phone. Does the leather pouch have the same feature ? I would like to try it if it does.
Xantris clear acrylic case and a NiteIze BlackBerry Rugged Holster. It has outlasted my Curve, and now comfortably protects my Tour. It still looks almost like the day I took it off of the rack, and has taken some serious abuse and never allowed me to worry.
Posted from my CrackBerry at wapforums.crackberry.com
I have a Storm 9550 and I carry it in a silicon case. When placing the phone in my pocket I have it wrapped in a microfiber cloth. I know it sounds crazy, but I get all the benefits of carrying it in a holster and it keeps the screen really clean having it wrapped in a cleaning cloth when in my pocket.
