iOS5 has an issue which periodically causes the iPhone's cookie settings for Safari to be changed unbeknownst to the user. If this occurs, we try to display an informative message to the Jive Mobile user but there are times when the browser is in a state which prevents that message from displaying. As such, it can result in the Jive app not accepting your username and password or avatars failing to load. If you encounter such a condition, go to Settings > Safari > Privacy and make sure it's set to accept cookies either From Visited or Always.
Rumor has it that Apple has fixed this in 5.0.2 so this should just be a temporary annoyance.