The biggest problem I foresee with a mobile client having thought about it, is that you could loose a certain amount of anonymity.

If the carrier knows your location and can recognise or at least guess at certain traffic been bitcoin, then when you start the app to make a transaction to another mobile user in the same location it may be possible to tie the two together.

The same even goes for using a mobile website. I think ideally you need something like Tor running on the device to avoid the such a problem, and it needs to be running all the time really or your internet activity when you fire it up for a transaction will be a dead giveaway.

Of course this is only a significant problem when bitcoin gains enough of a foothold to cause governments to take notice, and hopefully by that time it will already be to late