Having used multiple virtual desktops with X11 (until moving to OS X), and then CodeTek VirtualDesktopPro (until that died for lack of updates), and then VirtueDesktop (which crashed all the time), I'm please to see this functionality integrated into Leopard as Spaces. But there seem to be some pretty obvious possibilities omitted from Spaces.app that have been standard in such environments for a long time. In particular, shouldn't it be possible to assign different backgrounds to different spaces (à la CTVD), in part to remind you of where you are? As far as I can see, it isn't even possible to customize the names of spaces: "1" "2" "3" ... doesn't really help a lot in remembering where you put things. There must be a string somewhere that could be modified, but I haven't found it yet.
Spaces also seems to switch you out of the space you're in much more often than other such apps, and I wish I could cut that down, but I'd be happy if I could at least assign my own names and backgrounds to spaces.