When I switched from my 3G with it to my 3GS with a restore from backup it was there and no matter what I did it wouldn't go away. I did have the icon with my 3G on 3.0 beta. In some cases I think it can pop up in 3.0/3.0.1 but it isn't meant to be there with the AT&T 4.0 carrier file. 3.0 wasn't designed to not have the camera icon so there is always going to be a chance that it could show itself.
There have been many reasons from employees that are in these forums and then no real answer from AT&T. It is all speculation of why it took so long. One was that they had to remove a code that opted us out from MMS for every 3G and 3GS, another was that they had to do system upgrades "so we have the best experience with out iPhones", and that Apple and AT&T held the information from the techs that would need to make the changes. But from the class action lawsuit that came out in Louisiana against Apple and AT&T, it looks like AT&T still hasn't upgraded the towers for mms which sounds like BS because every other phone uses it.
And the fact that I had it working great for 4 months on my 3G using 3.0 betas with earlier versions of the carrier file (and so do many others on their 3G's and 3GS's currently) showed that it obviously works on their system and they most likely could have had it working by now. I really doubt it should have taken this long since mms was announced in March giving them plenty of time to come up with a plan to get it done. I'm not saying it had to be done for launch but shortly after, it's mid-august now and they still don't have it. But I will give them until the last day of Summer which is September 21st before they hear from me and many other iPhone users because they promised late/end of summer.