I owned a Treo 600 and traded with IB's web-based TWS and chartbook.com's free, delayed emini charts.
It was far inferior to my usual setup, so I used it only as a backup. Even if IB were to offer a full-featured Java client for the Treo, it would be too hard to execute the fast trades on such a tiny screen. The data speed would be fine, since the Treo features GPRS.
In the end, I sold my Treo, bought a Sierra Wireless Aircard 750 (GPRS) mobile data card, and trade to this day using a small tablet PC and my T-Mobile connection. Even though GPRS speeds are like those of a 56K modem, they are sufficient for emini real-time data (provided you aren't downloading large files at the same time). I can and do trade from coffee shops, in my office, at home, and in the car.