said by ConstantineM:
is it at all likely that they will disable an account that's using 300MB/month through GPRS?
I think not not enough cases for them to look for it and I guess the GPRS network is not swamped.
What I would worry about it that the vendors are re-farming the spectrum and moving 2G that might mess you up.
I like SMS as the transport mechanism, the reason is SMS will probably roam much better than GPRS.