Yeah, I wouldn't go as far as saying he's any more honest than anyone else in Congress. This decision seems to be born of the same frustration we all have at Congress not being able to get anything done, plus his possible frustration at being passed over for the VP slot on Obama's ticket.
Hopefully a scandal isn't in the works, but you never know with these guys. I'm not surprised the Republican Party is already spinning it as Bayh being afraid of his potential re-election competition. What should be coming out of their mouths is "We understand his frustration, we apologize for our part in government's ineffectiveness, and we'll take Bayh's decision as a call to action for both parties." Fat chance of that.