Ubuntu is based on Debian and Linux Mint ( just Mint ) is based on Ubuntu. But Ubuntu changed a lot of stuff from Debian, which is why ubuntu and debian things don't always play nice, even while they are the same base.
Linux Mint Debian Edition follows Debian Testing, hence why there is no version numbers, testing is always testing updates just roll out, except for right now as Debian testing is in a freeze getting everything wrapped up nicely to ship out Wheezy. At whcih point, updates will start being more frequent again.
Edited by Shrak - 11/1/12 at 2:19pm