With monstrous phones pushing 5" screens (here's looking at you, Dell Streak), these behemoths don't always fit too comfortably in your pockets anyway, and getting them in and out is a pain, especially if you're sitting down.
Won't Break Your Phone
Even as far back as 2004, Siemens conducted a study showing that squeezing handsets in tight jeans was one of the most common reasons for broken phones (the most common being dropping it on the floor).
Vaja's immensely beautiful leather cases are absolutely gorgeous. Their neat clip system allows you to unclip the entire cover as well, so the phone stays protected.
Holsters and Gadget Access from the Unplggd Archives
Case-Mate Phone Holsters
Case-Mate carries a large line of cell phone holsters (iPhone samples linked), ranging from horizontal and top loaders to arm bands for the active user.