Don't quote me on this, but I could have sworn that I read somewhere that Time Machine requires that the ENTIRE backup hard drive be utilized for backing up one drive with Leapard running on it. In other words, to have Time Machine back up your Macbook Pro, you'd have to use the entirety of one of your externals. I'm also pretty sure it wouldn't let you use partitions with Time Machine. Again, it has to be the whole physical drive.
I could be wrong about this, but that's what I remember reading.