I've noticed this once, the issue started when I connected via 3G and started driving around town. At one point at a red light, I noticed that I couldn't do anything on the net, like check email or surf, but I didn't really need it and left it like that.

After a couple of hours I needed to make a call and that's when I noticed that I can't do that, so I had to reboot. After the reboot everything started working and I got several texts that I had voice mails.

From this anecdote I can conclude that you shouldn't be driving around with GPRS on, or your GSM chip/software might lock up and you will not be able to receive or send calls and texts.