Since this is all conjecture anyways, I've got a couple theories that could support why Bo would resign.
1- He was basically blackmailed after tapegate, to prevent being let go at that time, maybe he had to agree to resign if certain milestones were not reached this season. It has been rumored that he was required to either get to the CCG and perform well or win it. His demeanor these last few weeks has been extremely subdued imo. It wouldn't surprise me if maybe he has been anticipating this and dreading it for quite some time.
2- Bo seems like a pretty straight shooter and standup guy. Maybe he did not meet some performance standards that had been set and is actually holding himself accountable for falling short.
3- Possibly the rumored 2nd, unreleased, tape recording of him going off at a practice is being held over his head, to assure a resignation and to prevent the distasteful firing of a 9+/- win coach. I would think that would be something the Nebraska AD would be highly sensitive to after the Solich deal.
Anyway, I don't think Bo resigning is near as farfetched as some here do. If true, I've got highly mixed feelings. A few weeks ago, I might've just said good riddance but I've seen much these past two games to renew some hope. One things for sure, this rumor sure does match up with some other recent rumors about Tressel being interested, etc. The fact that this guy, whoever he is, put his name to it, knowing the potential sh#tstorm, only lends credibility imo.