While looking for something else in my running process list in Terminal over the weekend, I noticed something odd in the output of the ps -A command -- a whole bunch of identical processes, apparently dead but not gone, related to AddressBookSync. In the ps output, I had at least 30 processes that looked like this:
....
46487 ?? 0:00.00 (AddressBookSync)
46589 ?? 0:00.00 (AddressBookSync)
46649 ?? 0:00.00 (AddressBookSync)
46710 ?? 0:00.00 (AddressBookSync)
.... etc.
A quick trip to Google found that this is a known issue with Mail in OS X 10.5.5 and newer. Basically, for every message you send, Mail will create one of these dead processes.
If you leave Mail running for an extended period of time, and send a lot of messages, this will eventually create a big problem, because you won't be able to open any applications. When you try, you'll see a message stating that "unexpected error -100008 occurred." A friend told me that if you exceed 120 of these zombies, you'll see this problem. I haven't confirmed that exact count, but I thought there was a 100 process limit in OS X, as explained in this much older hint.
To see how many dead AddressBookSync processes you have, just run this command in Terminal:
ps -A | grep [A]ddressBookSync
If you've sent any messages in Mail since you launched it, you'll see one entry for each sent message. The solution is simple, if annoying -- just quit and relaunch Mail regularly. Each time you quit Mail, the system will clear all the dead processes. I hope this will be fixed in a future 10.5 dot release, as it's a fairly major and annoying issue.
[Update: I modified this hint to remove the references to zombie processes, which I took from the linked Apple Discussions forum. As noted in the comments, these processes aren't really zombies.]

