I now have completed a first version of a reimplementation of 
WeakKeyDictionary, with the objective of improving performance
(both computation and memory consumption). The entire change
with a discussion can be seen at

http://bugs.squeak.org/view.php?id=6348

In essence, this introduces two changes:
- when values are finalized or keys removed from the dictionary,
   it is not always rehashed anymore. Instead, the dictionary just
   releases the value, and marks the association as expired. Expired
   associations can then be reused if a new value is to be inserted
   under the same has. Rehashing is only done when so many entries
   have expired. Expired entries don't count towards the size of the
   dictionary, and aren't reported when iterating over the dictionary.
- WeakKeyAssociation was rewritten to be a weak class, with a fixed
   slot for the value, and a weak slot for the key (currently, it
   has two fixed slots, with the key slot referring to a WeakArray
   of length 1).

In some measurements involving large (10000 keys) or many (10000)
small weak key dictionaries, this new implementation improves 
performance by a factor of 4..5, when measuring finalization time.
The second change saves 8 bytes per association (on a 32-bit
machine).

Regards,
Martin