Walther P22 is pocketable (it fits a blackhawk #4 holster), and some are reliable. If you insist on .22, demand pocket-carry, and won't do anything but semi-auto ... you get 22LR and lose out on 22 magnum options. You also rule out the most reliable 22LR pistols (Ruger mkI/II/III or 22/45 and Browning Buckmarks, plus high-end target models), so I think you are left with the P22, heavily tested, tuned, and loaded up with Stingers from CCI or another high-end 22LR hunting ammo of your choice.
All that being said, I have a P22, and I have taken to using it for point-shooting practice ... it is accurate enough for close-quarters defense, when it doesn't stovepipe, fail to return to battery, light strike, etc etc. To be fair, I haven't done the modifications/tuning laid out in the P22 bible (search here for it, or WaltherForums has it stickied), because I just let the P22 be a malfunction trainer for full-power pistols, cleaning and lubing it as well as I can, but not going to any herculean measures to tune it up.
Now that I think of it, the P22 with 10 rounds is the same size as my Walther PPS with the 6 round magazine ... if you are supremely confident in your shot placement under stress, I suppose there might be some merit to 10+1 (minus one malfunction) of 22LR when compared to 6+1 of 9mm. I am not that confident in my shot placement, so I opt for 9mm and practice reloads (on both P22 and PPS, they have the same oddball mag release).
If the NAA minis weren't so damn tiny, adorable, (and I assume more reliable) I'd seriously consider the P22 as a BUG, but I think if I ever reach the level of tacticalness that requires a BUG, the mini-revolver will be the way to go, in 22 magnum.