Wow. Yeah, *some*thing's changed recently to cause this. If I had to guess, I'd say it's a recent Thunderbird upgrade that's triggering it, as it's been 2-3 weeks since my last dovecot upgrade.
Some other interesting data points:
This problem seems to happen when I use Thunderbird (both on Linux and Windows) as my MUA, but not with Squirrelmail.
The IMAP process seem to start going haywire when I shut Thunderbird down.
Thunderbird would use the inotify stuff, but I don't think squirrelmail does since it's web based.
On the installation where a pinned process is, the 2 people accessing that maildir are using Outlook. They both use Outlook.
One of them has the specific version: Microsoft Office Outlook 2007 (12.0.6316.5000) SP1 MSO (12.0.6320.5000)
And I'm sure the other one will have some version of Outlook 2007, but I haven't been able to get in touch with them yet.