The mobile internet is the future. People will be able to receive IMs and emails, photos and RSS feeds, software and updates on their phone, while they are out and about. Chances are that the prices will come down, too. Flat-rate data plans overall work out pretty well. I've been using one on O2 for some time, and it's quite acceptable. Windows Live Messenger and push email are two definite positive points (being a Windows Mobile smartphone user). But where do the technical limitations draw the line?
On a good day, with a good signal with an EDGE connection, you are still looking at least 500ms ping. That can go up to an average of 700ms without EDGE (so instead, plain GPRS), and up to 1000 to 4000ms on lower signal strength. Obviously a second or two doesn't matter for your IMs or emails, but streaming of any kind is pretty difficult. 3G obviously was created for this purpose, but don't expect all of those non-3G handsets to ever do something like online radio too well. (Youtube via EDGE is bad enough.)
Mobile connections still aren't good enough to support a lot of more bandwidth-intensive or time-critical applications, but mobile connectivity is certainly progressing. If not, there's always WiFi, right?
