There is an oddity in Google's IMAP implementation.
GMail does this thing where if you send a message to a list, it doesn't show up in your inbox (even though you are a member of the list). It's documented here. Your message will "appear" in your inbox as one of the "threads" when someone replies.
That works all fine and dandy in the web interface. But using Thunderbird (or any IMAP client), it means that none of your emails to lists show up.
This is especially apparent if you are using a "threaded conversation" view in your fat client. There are gaps in the thread where you are the one emailing the list.
I kinda wish they would tweak that. Not in the web view, mind you, in there it works great; but when accessing mail via IMAP (or POP3, I would imagine), it would be nice if it went ahead and communicated the messages you've sent to lists.