Wouter's post prompted me to write this...
I find it particularly strange to have a single filesystem for everything, especially so on a laptop. I worked with a guy once who had the single filesystem (I think it was ext2 or ext3), and his laptop crashed, and the filesystem was trashed so badly from the crash (I remember: it was a PowerBook, so I don't know if that had anything to do with it) that it was a write-off.
One of the reasons I use multiple filesystems apply just as much to a laptop as to anywhere else: loss/damage compartmentalisation. If one of my filesystems decides to eat itself, odds are I can still get the others back.
Wherever I go, I tend to have a root filesystem, one for /usr, one for /var, one for /tmp and one for /home. If it's a server, I may have a separate one for /srv as well. I always use LVM, and with ext3 supporting online resizing these days (well online enlarging at least), it's not a big problem to leave some logical extents unallocated, and just grow whichever filesystem needs it as it needs it.