I get the same problem when I use APNdroid. An app that lets you switch on/off GPRS at will.
Except while it switches GRPS off, ok, on my phone it never switches it back on. I have to Power OFF/ON to enable it.
I think re-enabling GPRS after disabling it, is a bug with the Galaxy Lite/Spica/Portal.
Phone: Galaxy i9100 SII: Now using Galaxy TAB7.7. Sowheat!!