I know my prot-pally has an entire 20-slot bag of tanking gear he carries at all times, and I do not envy the gearing challenge of druid-tanks who often carry three entire sets.
Enterprising players start their own guild on a bank-alt and buy bank-slot tabs. And there's no way to give tanks extra space without giving it to exeryone. There's been hinting that bag size may not increase beyond the current max of 20-slots into Wrath. Unfortunately, Blizz has said that the repurcussions of dramatically increasing bag-space on server memory is what's preventing them from allowing players to have larger bags and banks.
