According to Saul Klein, mentors are allowed in.

From: http://blog.seedcamp.com/2009/09/awaiting-moderation.html

"One quick factual point – of course mentors are able to attend the judging sessions, several did this year and have done every year. I think what happened yesterday was a pretty innocent misunderstanding – you came into the room mid-pitch and as you’d not been a mentor during the week, I guess that’s why Reshma asked very politely for you to not join the session."