2 Apple IDs on one device? Can you have multiple Apple ID's on the same iOS device?

My girlfriend and I have both got an iPad and an iPod Classic and may well be getting iPhones at some point, we each have our own apple ID and I was wondering if I could have both Apple IDs logged into one of our iPads to avoid us having to buy Apps twice.

I would have thought that Apple wouldn't allow this, but I just wanted to check as I can do this on my current Android phone (Two Google accounts rather than Apple IDs obviously).
Yes, you can have apps installed from different Apple Id's.  Simply log in with whatever account you both want to use for purchasing.  You can log out and back in with a different App Store account at any time.  

I have multiple accounts, and multiple devices sharing applications.

To follow up the other responses, your App Store account can be separate from your iCloud, Messages, and FaceTime accounts.  I use a separate account (Apple ID) for iTunes purchases than my iCloud account. 

Some people even split things the opposite way - sharing email, FaceTime and messages but having different store accounts. It's really up to whatever works best for you and your devices.