I just enabled this feature (using WordPress 2.6.3) and I’m not getting that error.
However, the body of my e-mail isn’t showing up as the body of my post–the body of my post is empty. The subject is mapping fine; I’m e-mailing from a Gmail account.
EDIT: Oops…I didn’t see this was about the beta version; I was just searching to find a solution to my problem and this post was the first one to come up. LOL.