Lexar (and a few others) came out with these Expresscard Solid State Disks (basically flash memory in Expresscard form factor) this year, apparently to sell them to Vista Notebook owners as a way to take advantage of Vista's ability to use flash memory as a disk cache. For example,
http://www.newegg.com/Product/Product.aspx?Item=N82E16820191058
It seems like this should also work as a small volume for TM on a MBP. I typically only backup my Documents folder daily, and archive the rest of my stuff to DVD's only when they change. I put everything I work on in Documents (code, text files, PDF's) so it never gets that big. Since these Expresscard SSD's apparently fit flush into a notebook computer, it would more or less (for me anyway) solve the problem of dragging around an attached external drive with my MBP in order to use TM. The 4GB model is < $60, but they are available all the way up to 32GB (for a price).