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Wirelessly posted (iPhone 3GS: Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.4 Mobile/7D11 Safari/531.21.10)

You mean none at all? Of so check settings network cell data.
 
I've had an error message pop up once or twice, something about cellular data something not active but that was literally only like twice and that was when I was browsing the web, sorted itself out within a minute or so too
 
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.4 Mobile/7D11 Safari/531.21.10)

I have also noticed this problem. I have seen error messages stating that no Internet connect could be detected in many apps.
 
Same problem. I have good reception and a nice 3G icon in my status bar but no connection. Cellular voice and messages work fine though. Wifi works great.
 
Same problem - solved?

I think it's Apple being too clever.

I use a BTmobile network that is provided by Vodafone. When I upgraded to OS4.0 the installation programme detected that I was with Vodafone and rewrote all of my BTMobile settings for WAP and MMS to Vodafone defaults.

When I changed them back it was fine
 
I've had the same issues, never really noticed it since im on wifi alot though. But the 3g speed is incredibly sporadic, normally for the worse. This was not the case till i upgraded to 4.0 on my ip3g, or since i got the ip4, my bro's 3g still is fast with the 3.1 os.
 
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