With the release of the new OS X there was an announcement of better support for multiple displays. Is it possible to hook up an iPad onto your mac and use that as a 2nd display or only your mac and displays?
This isn't quite true. It's not as easy as plugging the device in to your computer and it's a bit kludgy, but it can be done.
I've seen a few guides touting different programs, but the basic idea is the same: get a VNC client for your iPad or iPhone (these can be had for free). Then get a program like ScreenRecycler (not free), which allows you to take control of a monitor on a networked computer running VNC or, in this case, your iPad/iPhone.
That sort of solution is decent if your systems are on a wired network together, but the iOS devices can only connect to networks wirelessly. It works, but the refresh rate is a bit sluggish. If you just want to put information up on a screen and don't require much animation or interaction, it's a decent solution.