CI Sessions the guide! Maybe...

[eluser]GSV Sleeper Service[/eluser]
codeigniter sessions are weird, I experienced a fair few problems with them, mainly with IE 7 (for reasons still unknown, must do some more testing)
I recommend the 'native session' library, which you can find in the CI wiki, you lose the database stuff, but I can live without that, and they act just like regular old PHP sessions, ie none of this stuffing everything into a cookie oddness.