WordPress MU

I have a several year old MU install with a couple of hundred blogs and users. Recently updated to 2.9.1 from an very old version, and since then one of our users (one of the earliest ones) has his comments consistently being marked as spam even on the blog that he is admin of. When he goes to comment the comment from shows him as logged in but we have to go manually approve his comments to get them out of the spam queue.

Is there some way users are marked as spam across the board that I am unaware of? His comments don't even seem to be that spammy - not ones that would generally be caught by a spam filter, I think.

And actually it turns out he's not the only long-standing user tat's having the problem.

Hos anybody else seen this, or does anyone have a pointer that miight start me down the right path?

Thanks!

Followup:

I did check in wp_users and he is not marked as spam there.

Is he replying to other comments while logged in to the admin area?
Because this is happening to me too and it's annoying the hell outta me.

For now, just tell him to reply from the front of the blog.