Can a money purchase pension plan permit in service withdrawals after the attainment of the plan's normal retirement age? Isn't there a revenue ruling that discusses this?

Yes, I believe that the consensus is that such distributions would be allowed after NRD.

Don't have a cite handy.

However, you can look to the fact that the IRS specifically allowed all Plans to keep mandatory distribuitons to currently employed partcicipants (non-5% owners) who reached age 70 1/2 even though those distributions are no longer mandatory. In essence, allowing a Plan to "mandate" in-service distributions.

You may want to look at this link:

http://www.benefitslink.com/boards/index.php?showtopic=7054

By definition, an in-service withdrawal is one that is taken when there are no triggering events. Attaining retirement age as stated in the plan, is a triggering event. A distribution upon attaining retirement age is not considered an in-service withdrawal and is therefore permissible in an MPPP.

Clarification:
"A distribution upon attaining retirement age is not considered an in-service withdrawal and is therefore permissible in an MPPP. "
That should be Normal retirement age, as defined in the plan.

Also, be sure to check your document. Many condition the benefit on actual retirement and not merely attaining NRA. Also, I have seen a number of MPP prototypes that do not even provide an option for in-service distributions after NRA so you may have to restate to one that does, or go the individually designed route.