I just had the problem of exporting a folder from my gmail account to disk so I can process the emails further. Unfortunately I did not find an export function in gmail. So gladly GMail now supports IMAP (if it does not work for you and you don’t use the english interface, try to switch your gmail settings to english, worked for me). Now I also can read my gmail account via Thunderbird but Thunderbird does not have a export function as well, you can only save individual emails.
After some digging I found an Im-/Export extension for Thunderbird which you can find here. It allows you to download a complete folder in various formats, EML, HTML, Plain Text, Plain Text in one file or just the index. It seems it can also process a whole account. Of course it also allows you to import your email again.
I thought I blog it should it be of use to you :-)