"John Harlow"
time). The approach controller assigned me a transponder code and
told me "after departure remain clear of the class C airspace".

Lol - in my experience, either the airspace is too busy - or you sound
like
you could be a nuisance.

Sounds like it could have been a standard instruction for any a/c on the
ground calling in before departure. Not clear anyone is a nuisance or that
there is *any* traffic. Local practices vary.


. Well, the controller then called
me by my tail number and asked some questions (I don't remember
exactly what -- it might have been my expected cruising altitude and
aircraft type,

If you didn't automatically give him all that right after establishing
contact with him (assuming him), then maybe that's why he didn't want to
be
bothered by you. I can just hear the controllers sigh when someone gets
on
and says "podunk approach, november 12345, would like flight following"
and
then the game of 20 questions starts.

Did the controller contact the pilot after departure before the pilot called
himself?

to be sure that I understand this for the future. So ... if I'm told
to remain clear in the future, WHEN does that end?

When you explicitly get permission to enter.

.... or in this Class C situation, after announcing my intent to proceed
direct on course thru the Class C. I would consider any acknowledgement of
the call that did not include a "remain clear" to be sufficient to proceed
on course. That's not how I would do it, but I think that would be ok.

I'm thinking that it may be local practice to instruct any pilot on the
ground to remain clear of the Class C. Once in the air, the normal Class C
procedures would apply. Which would mean once contact is established, entry
would be permitted subject to any instruction to the contrary. But since a
remain clear had already been issued, I too would want explicit permission
to enter.