I have the same problem with that card and encryption on my Slackware 9.0 distro. When running 'iwconfig' as root, I would at first see the settings, including encryption.
Then, at some point, I would lose the connection (not to mention it was very slow to begin with). Then I would run 'iwconfig' again, and looky-looky, all the settings were gone.
Perhaps its something with the Linksys?
The only solution I've had is to turn off encryption.