Hi Arden,
This is a periodic problem specific to IE. It seems to be more common in IE6 but we've seen it in IE7 as well. You can work around this issue by clicking Tab, or clicking on the "source" button within the editor, then clicking it again.
We've been unable to consistently reproduce the problem, but have implemented a number of changes to try and permanently resolve the issue. We're currently trying to reproduce in 2.2 (just released) and so far haven't been able to - though that doesn't mean it's fixed. We'll keep looking into this until it's resolved.
In the mean time, if you have a sequence of events that reproduces this, let us know.
Thanks