Further to this, I found this in the ATC manual...

http://www.faa.gov/airports_airtraff...atc0403.html.1

"Hold for release" instructions shall be used
when necessary to inform a pilot or a controller
that a departure clearance is not valid until
additional instructions are received.

While it's grouped together with void times and other techniques used
at non-towered airports, there's nothing in there at explicitly says
it can't be used to indicate an incomplete clearance at a towered
field. Still it's awfully misleading. "I'll get back to you with the
beacon code" would have been a lot clearer!