I have been having this problem for bsolutely months now with my iPhone. Now 3 phones later (3GS), new sim card, new number and some network fiddling with my account by O2 I am still having this problem. What happens with me is if someone send me a long message, usually if it is over 4 texts or longer (sometimes happens with just 3) then the phone wont receive it. This will then, 9 times out of ten, lock the phone up meaning I cant receive calls or texts or make calls or texts. I have to turn off then back on, or turn airplane mode on then off to unblock the phone. After apple not knowing what the problem was, and O2 not caring, I did some testing myself. When I restore the phone it is fine, when I install apps it is fine, it is only when the apps have been loaded up for the first time especially ones that save ur login in details such as facebook. Once they have been used, and closed down, they seem to affect the network and not let large amounts of data in such as a long texts. Now to me this is a carrier and Apple problem surely. Not allowing texts messages through cause apps are taking up to much bandwidth!!
I am in the UK and on O2 in the Portsmouth area. Strangely though, this problem doesn't happen when I am in Bognor. It is almost as if there is less people using the bandwidth there so O2 have more space to puch the messages through despite the apps restricting the amount.
Any thoughts? I think this issue needs to be made more public for networks and Apple to look at!