I'm surprised no one has mentioned the gadget shoulder holster:
http://www.thinkgeek.com/computing/bags/918a/
combined with the gadget hip holster:
http://www.thinkgeek.com/computing/9190/
Also useful for carrying a can of mace, which you can use when you're mugged and/or assaulted.
Edit: I forgot about the grand daddy:
http://www.thinkgeek.com/gadgets/cellphone/c616/