I've always been curious...

Let's say I run Kubuntu and I want to try Gnome or IceWM, is it possible to have two or more WMs installed at once and just pick and choose depending upon my mood?

Will running all three put things in different places? Or will they all just reference the same data pretty much as far as where things are in system?

totally possible yes. most X login managers (you'll presuambly be using KDM) wil loffer a drop down list of desktop for your preference at the time of login. window managers put their own data whever they feel like, but the applications runnign within them run just as normal, the window manager has no influence over them.

You can run many different window managers or desktop environments. Off the top of my head, I've got blackbox, fluxbox, kde, IceWM, twm all installed, although I only use fluxbox really.

What's more, if you like, you can run two instances of X with two different WMs or DEs simultaneously.

edit: beaten to it!

Or 3---or 4. Actually, is there a limit on the number of X-sessions that can be running?

I guess you're limited by the number of ttys, so presumably, you can have only 6 going at once, using the default setup. Obviously, you'd need a lot of computer to run that many at once!

So, you just switch between TTYs and you go right into the open session? Different graphics and everything?

With multiple X sessions running, you switch between them using ctrl-alt-F7, F8, etc.
In my setup (Mepis/KDE), you can simply select the session--or create a new one-- in the "switch user" menu.

Wow. That's pretty darn cool!

Why would one use multiple WMs? Especially at one time?

Just by way of completeness, since I don't boot straight to X, to start any X session, I have to do it manually. So, for me to choose a different environment is just a case of typing a slightly different command:

startx = fluxbox (default)
startkde = kde
startxfce = xfce
etc.

Although there's something wrong with startkde in that it doesn't work, but there's the potential!

I have used two in the past, and I can't quite remember why, but I think it was for something silly like changing the time on my machine. I was in fluxbox, and couldn't remember/think how to do it from the command line, so I fired up KDE on a separate tty, and just changed it from the clock in the bottom right hand corner.

I suppose you might do it if there were things which didn't work well in one, but that you needed to use simultaneously, so you fired up the other. Like I said, it's pretty resource intensive though!