I switched over to using only my laptop in January, and had exactly the same question. As far as I could tell at the time, the answer is no. Let Photoshop use the boot disk for scratch, and it will optimize it. Just make sure you have plenty of empty space -- once the boot drive is more than about half full, it slows down a lot.
Having two partitions means the head will sometimes be swapping back and forth between them, which is very slow.
A Firewire 800 external drive that you might use for files and backup can be partitioned with one 20-50GB partition at the fastest outside part of the drive -- you can then use this for scratch. I'm not sure how it compares speed-wise with letting Photoshop just use the boot drive, but a lot depends on that boot drive (capacity, how full it is, etc.)
Sorry I can't provide any links. I searched like mad to find anyone other than Mac Performance Guide that recommended partitioning the internal drive in a laptop for Photoshop scratch, and couldn't find anything in support. One of our staff tech guys suggested that a partition for applications and boot at the *slow* end of the drive might make sense -- that is, create a large fast partition for data and scratch, and a smaller partition at the inside of the drive for boot and applications, which only have to load once. Haven't tried that.
Good luck.
