I got it to work again, after letting the phone & tablet sit for a while. But when I disconnected, I couldn't get it to reconnect until it sat for another couple hours. So I don't know if it is an issue with the tablet (probably not), phone, or t-mobile. Even after a fresh reboot of phone & tablet, it still doesn't work. My guess is that t-mobile still thinks the session is open on their end and I have to wait for it to reset before I can use it again.. Does this sound plausible?

My other option is to try and establish a point-to-point network connection between the phone and the tablet, then use NAT on the phone to forward internet traffic (the A780 phone runs linux). This always works reliably between my PC and the phone using a usb connection (but I never could get usb dialup networking to work). I just have to figure out how to set up a bluetooth lan connection.