MySQL is pretty close, particularly 3.x. Just make all your columns VARCHAR(255) and when you treat them as numbers MySQL will autoconvert. If you want to allow for bigger fields just make them all TEXT, which should work (although I've never tried treating a TEXT field as a number).

I agree that a typeless relational database would be a very useful thing to have. I might not use it for the most critical projects but it would make one-offs a lot easier to develop.

-sam