I just recently setup Thunderbird 2.0 to show my Gmail account over IMAP using the excellent tutorial here. However, as others have commented, the original instructions for deleting messages to the Trash folder on Gmail from TB no longer seem to work. I have found a simple way to get it working again.

The original article tells how to create a new preference string in Tools -> Options -> Advanced -> General -> Config Editor. First, find the number X in the section mail.server.serverX that corresponds to your Gmail account, then create a new string, mail.server.serverX.trash_folder_name. Here is where the change is. For the string value, use this syntax:

imap://userid@imap.gmail.com/[Gmail]/Trash

where "userid" is your Gmail account name. This is working for me. Post back if it works for you, too.