No they don't of course. But Linux kernel has a nice way of dealing with apps that misbehave, using POSIX signals. This is enough on desktops, and should be enough for handhelds as well. Also, the device drivers need to be written in a way that a misbehaving app cannot crash the entire system.
The other thing is that apps should not be able to change system files. For instance, a while ago I had installed a vpn client on my N810, and tried connecting to work server just for fun, but it ended in changing the /etc/resolv.conf file, thus disabling any internet access. Imaging something like this happening to someone who's used to things just simply working!