Info on sub keys?

[Felix E. Klee]

I've a couple of newbee questions concerning sub keys:

* Aside from convenience, is there any difference between a sub key and
  an ordinary key signed with the master key?

* Can such an ordinary key be transformed into a sub key?

* Since when (date and version) does PGP and since when does GnuPG
  support signing sub keys?  I ask because I read that old versions, at
  least of PGP, support only encryption sub keys, not signing sub keys.

* Are signing sub keys part of the OpenPGP standard?

* One can include any number of sub keys into a key, right?  I ask
  because I recall reading that there was/is some problem with key
  servers and sub keys.

If there is any good documentation on sub keys, aside from technical
specifications (such as RFC 2440), then please let me know.

-- 
Felix E. Klee