This is my first attempt at making hashcash.el asynchronous.  It
doesn't address all the ideas brought up in the discussion here, but
it's a good start.

User-visible changes:

* New function mail-add-payment-async takes all recipients in To and
  CC headers and starts generating hashcash tokens for them
  asynchronously.
* New function hashcash-cancel-async stops all asynchronous hashcash
  processes for the current buffer.
* New function hashcash-wait-async waits for all asynchronous hashcash
  processes in the current buffer to finish.
* New function hashcash-wait-or-cancel does one of the above depending
  on user input.
* hashcash-default-payment and hashcash-default-accept-payment are now
  20 by default, as recommended near the end of
  http://www.hashcash.org/dev/ .

Recommended hooks:

(add-hook 'message-setup-hook 'mail-add-payment-async)
(add-hook 'message-send-hook 'hashcash-wait-or-cancel)

Unfeatures:

* Asynchronous processes are not started automatically when you enter
  new addresses; you have to type M-x mail-add-payment-async
  yourself.  It seems that an idle timer, checking for point being
  outside of the header (to prevent tokens for halfwritten addresses)
  would be the solution.
* Incrementally increasing the collision length while the message is
  being written is not implemented.  Strictly speaking this is
  impossible, as the collision length is part of the hashed string,
  but this could be emulated, calculating longer and discarding
  shorter tokens.  I'm not sure it's worth it, though.

Bugs:

* If you call mail-add-payment{,-async} while a token is being
  generated asynchronously, you will get duplicate tokens.  Tokens
  already generated and inserted in the buffer are not duplicated,
  though, using hashcash-already-paid-p.

Changelog:

2004-11-02  Magnus Henoch  <mange@freemail.hu>

	* hashcash.el (hashcash-default-payment): Change default to 20
	(hashcash-default-accept-payment): Change default to 20
	(hashcash-process-alist): New variable
	(hashcash-generate-payment-async): Add
	(hashcash-already-paid-p): Add
	(hashcash-insert-payment): Don't generate payments twice
	(hashcash-insert-payment-async): Add
	(hashcash-insert-payment-async-2): Add
	(hashcash-cancel-async): Add
	(hashcash-wait-async): Add
	(hashcash-processes-running-p): Add
	(hashcash-wait-or-cancel): Add
	(mail-add-payment): New optional argument.  Conditionally start
	asynchronous calculation.
	(mail-add-payment-async): Add

What do you think about this?

Regards,
Magnus