Yes I also have this issue, most the time though it seems that if i wait for a little bit it will let me in but I have had to delete my account then re-enter it for it to work but tonight it seems to let me log in just fine... weird.
Not to go off topic but does Mobile IM go against your data or text plan? I've never used it but I know a window popped up when I went to check it out that stated something about some programs may use your text plans or something to that extent.