PBJ in traffic court shouldn't be compared to probation on a criminal conviction. PBJ is an alternative sentence offered for first time speeding offenders so that the charge doesn't appear on their record. I doubt that a PBJ for speeding will result in a denial since that isn't the intention of 8 CFR 316.10(b)(3)(c).
Hmm ... but like an unpaid ticket, this probation means the resolution for the violation is still incomplete. And they normally won't let you naturalize if you have a ticket which is unresolved because you have an upcoming court date or you haven't paid it yet.
It is also possible that the ticket was classified as a criminal offense (misdemeanor of course), given the length of the probation and the amount by which the speed limit was violated. In some jurisdictions going 15 or 20 over the limit is a misdemeanor.
Instead of blindly applying right now, I would suggest that the OP take one of these options:
1) Wait until about June 2009 to apply, which is 5 months before the probation expires. By the time the interview happens the probation should be done. If the interview is scheduled before the probation is completed, ask for a reschedule.
2) Get a one-time consultation with a lawyer to find out the exact consequences of this traffic offense and probation, then take action based on what the lawyer says.
3) Apply now. If they won't approve it because of the probation, ask if ending the probation by accepting guilt and paying any additional fines and submitting proof within 30 days would allow you to naturalize. Or if the interview date is late next year, close to the end of probation, ask for a reschedule.