Omen,
This is actually an error that you're likely to encounter every time Mail is updated. Normally, you would just have to reactivate the Mail rule in OmniFocus, but until we get the 1.10.3 update out, you need to follow the Mail Rule instructions here:
http://www.omnigroup.com/support/omnifocus-mtn-lion
Sorry to everyone for the bother and confusion. We are working as fast as we can to get 1.10.3 released soon.