In Linux you very rarely have to reboot. And it's getting better all the time...
Ubuntu has a new Init script system were it's much more intellegent about system dependancies (for example: you don't start file sharing until after you get a network connection) in services and such, so things can be restarted more intellegently without rebooting.
Currently the only time you _have_ to reboot is during kernel upgrades. And with kexec and such that isn't nessicarially true anymore.. You can swap out kernels on the fly (but it's not mature to the point were it's usefull for normal people. It's mostly for developers that need to examine a crashed kernel. The kernel craps out and boots up a miniture diagnostic kernel immediately without messing up the memory).
Restarting X is Linux's version of a reboot, realy. If you change the mouse around (and need special configurations for things like wacom pads, or 13button mice, or laptop pad) you have to restart X. If you wanted to add different monitor or change the aviable resolutions: restart X. Install new video drivers? Restart X. Want to hook up a projector? restart X.
And although it's not required.. when you do fairly significant upgrades to your system.. like a upgrade a bunch of Gnome dependancies when your using Gnome, it's probably a good idea to log out of the GUI and restart X.
At least that stuff is much quicker then a reboot.
In all seriousness I do see were it's easier to just to tell people to reboot sometimes instead of making them go through extra steps after a upgrade to avoid having their running programs crash or whatnot, evfen though the majority of the time this is not nessicary.
Has the RTM version of Vista solved the problem of making you reboot when you change the DPI settings? Or is it still doing that silliness? That's about the same level as 'reboot to change your IP address'.