The Cult of Mac Blog is reporting that since Apple began offering .Mac email aliases, the number of available accounts has been shrinking fast. Still more reason to get your own domain set up and run your own mail server. ;)
But as usual, I have a suggestion...
Part of the problem, if you choose to look at it that way, is that once an address has been used by someone, it is never recycled to another user. After all, you wouldn't want to set up a previously used account as your own and then have to deal with that person's spam now, would you? Speaking for myself, my .Mac account gets enough spam all on its own.
One thing that Apple could easily do would be to set up another domain level at .Mac for folks to use, for example, user@G5.mac.com, user@Supercomputer.Mac.com, etc. Also possible might be for some of the domain names that Apple has aquired over the years that redirect to apple.com to also be used as email addresses, such as firstname.lastname@example.org, email@example.com, originalPDA@newton.com, etc, etc.
Opening up cool new domains for exploitation, er, use, by .Mac subscribers can only serve to add to the value of the service, and would most certainly increase the pool of available account names nicely.