It all depends on how you can handle the content.
If your sites are pure text, then even with the restrictions DrTandem wisely suggests, go ahead with expansive layouts...
But when you deal with graphics and layouts based on them, like in this case:
www.musubiaikido.com.ar tell me how was I supposed to make it "expandable"? I wouldn´t have done it even when finding a solution, but let´s imagine for a while...
On the other hand I really don´t like expansive-ad-infinitum sites. They never look enough cool to me. Neither those which are configured already in 1024x768. Even when I have a 19" monitor with a 1280x1024 resolution. I use the browser as "part of" the screen, not at full screen. Anyway, this is a question of taste.
You will see that almost all pages handle a fixed width or at least a maximun size. Basically when you are dealing I repeat with graphics as core of your design. not even to mention when using flash. I choose fixed width and centered pages that will always remain in a harmonious place.
Abou PDAs and WAP access, I would recommend you that you make special versions for them in xhtml or wml... with their own design, regarding their current "primitiveness".
Just my opion!!! ;-)