Well I guess Sprint got wise to this. Both 4G and 3G will not register GrooVe IP now.
It does work on the Galaxy Tab!
I ran into a similar problem today where I couldn't register on 3G. I think it was my phone's data connection got into a screwed up state. I stopped the app, put my phone in airplane mode for a few seconds, then out of airplane mode and restarted the app. That fixed it for me. Might be worth a shot. Or try restarting the phone.