Treo 600/650 can access just about any site out there, but don't think they are available with Nextel quite yet. Once Sprint takes them over you should be set. Blackberry does probably work for Pinny and a few others, but the key thing to ask for is Java capability. I have a non-Java capable phone (in fact I am writing this response with it) and it sucks that some sites don't work with it. Still the fact is I get Skybook's half points and Pinny and Holly's reduced juice thanks to it so the moderate hassle is worth it, unless you are like JJ and don't think shopping is worth it.
Also allows you to be productive during long rides on the train as I am doing right now. Cost is only $15 more than phone service with Sprint, a great bargain.