Atomic CHECKIN of all checkouts of an activity

Greg has asked for the ability to tell the server that he wants
all checkouts against an activity to be checked in atomically.

A natural way to marshall this would be to allow CHECKIN to be
applied to an activity (since activities cannot be versioned,
this is unambiguous).  Or we could introduce a CHECKIN-ACTIVITY
method.

Any preferences (or objections)?

Cheers,
Geoff