I'm having this problem since upgrading to Mango.
If i lose signal, once it re-connects my data connection is completely dead. Flight mode on-off won't revive it. The only thing that works is a soft reset.
This is... to say the least frustrating. It was working flawlessly till mango. I've even hard reset it to no avail.
My sister's omnia 7 is the same and there are at least 3 or 4 others on the xda forums reporting the same problem. All of us are on Orange UK.