I'm expecting my brand new HP hw6515 to arrive any day now, which runs Windows Mobile 2003 (will be upgradable when they launch the 2005 version early this fall) and supports EDGE for about 250 kbit data-transfer. Since the phone-networks are different it may not be as applicable in the US, but you could check out what kind of connectivity it offers in the US. I live in Norway, and pretty much anywhere where people live I can get that kind of speed. (Norway is a country of 4 million people, so these places are pretty small).
It has to be combined with a cellular subscription plan, but it can be always on and offer streaming quotes and trading as long as your broker/data provider offers a java application. And thankfully we have an operator which doesn't charge per MB of data!