What do you want to do with the RPi?
You can run it with only a text console, and then you don't need a mouse, even if you still can use one. Just log in with the command
sshor use the computer without installing or starting any X11 server or xdm program that starts a X11 server on your computer, like
kdm does. You can still reach the computer and start GUI programs that show the result on your computer. Just connect with
ssh -X computername and start
firefox. Don't forget to start them in back ground by adding a
&-character efter the commands.
If you want a GUI (Graphical User Interface) running on the screen of the Pi, then it depends on which graphical environment you use. There are lots of different Window Managers that you can install and some are very easy to use without a mouse, while others are harder to use.
But most window managers have good support of keyboard short cuts defined. And as Adam Honoré wrote, you can use the keyboard arrows to simulate a mouse device.
The graphical interface is much more flexible in the X Window environment, which you usually have on Linux machines, compared with what you have on MS Windows machines, and even Mac OS X (even though you can run X11 on Mac OSX).