A FULL client on a mobile device seems unlikely for now for a few reasons.

1 communication costs. bitcoin is steadily sending and receiving data updates. It seems the mobile companies around here are going for higher data costs and fewer unlimited data plans (or none).

2 memory costs. the Satoshi bitcoin takes over 600 mb to run. Perhaps bitcoinj based client would be better, but how much better?

3 storage costs. Full block chain is currently 340mb, block chain index is another 150mb. wallet is variable size depending on activity. (I have one wallet over 1 gb.) Phones memory card or usb drive support make this go away but that leaves out a lot of phones.

4 battery life. with the constant communications related to point 1.

Of course all these are subject to change. in as little as 6 months it could all be a different story