Same exact bug for me (with a slightly different instance address) when opening an existing recent document.
"build Exception loading Group: -[Group disallowNotifications]: unrecognized selector sent to instance 0x101c43160"
I am on Mountain Lion DP4 (10.8 build 12A248). I just downloaded OmniGraffle Pro v5.4 from the OmniGroup website. Here's the build info:
Product: OmniGrafflePro-5.4.x
Tag: OmniGraffle/5.4/GM-v135.7
Date: 2012-05-24 12:40:08 -0700
Builder: omnibuild
Host: tb107a.private.omnigroup.com
Revision: 167456