i wish you could do that too- there is a PC solution that does exactly that but as far as macs go the closest solution (and it really isn't one) is Teleport, an app that allows you to control your laptop or powermac from a remote machine, which I love.
I use my powerbook for email, placed right next to my G5, and this app allows me to basically extend my workspace to the powerbook to check my email, etc. You can't move windows between macs, but you can copy the clipboard, etc. And you share the keyboard and any other usb peripherals, so you can type an email or letter, etc.
Oh, and it's free...
http://www.abyssoft.com/software/teleport/