I have one of those ultra-light devices. I tried to use it for trading (checking charts and looking for setups while on the move) and I just found it non-feasible. For trading you need peace and a confortable work space, not a 5-inch screen on the train back home.
I agree with the previous poster about needing piece of mind when trading. Trading is serious and deserves your undivided attention. That being said, I will sometimes watch the market on my iPhone using a website from CMS (http://www.iphoneforex.com) - it is nice because it refreshes automatically and is simple to use.