I don't use Ubuntu, but I saw this on CNET (link from Digg) the other day and it seems a possible workaround fix for your problem.

On Jan 13, 2008 10:53 AM, < fouldragon@aol.com> wrote:
>1-you mention that NetworkManager seems to "forget" your WPA key but
>other parts of your description sound more like the router has
>"de-authenticated" you.

...


--
Donn
There is a very fine line between "hobby" and "mental illness."
-- Dave Barry