Data Network mode doesn't get activated on gingerbread 2.3.4
Do not know if you got it sorted but I had the same problem and did this:


Tap Settings > Wireless and Networks > Mobile networks > Access Point Names 
Then tap on one of the relevant APN's pertaining to your provider, such as, for example O2 Mobile Web (Pre-pay or contract) 
Set the APN Type to 'internet,mms' 
Tap on Menu, tap on 'Save'.


Back out of the Access Point Names screen, back out to Mobile networks, then:


Tap on Packet data (In some ROMs this could be labeled as Data Enabled), and also, Data roaming to turn them off.
Turn those two options back on 


And voila its working again!