I bought a 16GB WiFi + 3G iPad in Stockholm and my data connection is with TRE (3). The data connection worked like the day I bought it and the next day it stopped working. Whenever I tried to access data, it showed me "Could not activate cellular data network". I restarted and reset the network setting but no use.Then I checked APN setting after reset it was bredband.tre.se I changed it manually to data.tre.se and bang, i am able to connect and surf again.
So if you have a TRE connection you many find your ipad plan not working when you have default APN setting. Try changing it to data.tre.se