Running PR 1.2?
Tried rebooting?
Tried putting the sim out and in again when off?
Possible that you might have over-used your dataplan? (Although most services only decrease speed after too much of use)
Installed many extras-testing or extras-devel packages?
Is the signal strength rich? (over 1 or 2 bars)
When you press 'control panel' > 'internet connection' do you see the service available? If not, look at your provider's site for phone settings. (Often are send to you for free via SMS)

Hope this helps