Cat-related IKEA hacks are always popular at Lifehacker, and this one is pretty neat: a combined cat litter tray and feeder area that ensures neither feline activity makes a mess of the rest of your living area.
Cutting a hole in the side of an IKEA cabinet is an approach we've seen before. The really clever idea is joining the two cabinets with a spare piece of panelling, creating something that looks like an ordinary cupboard. Hit the IKEA Hacker post for the full process (it uses a cabinet not available in Australia, but the same idea would work with virtually any single-door cupboard).
Akurum cat cabinets [IKEA Hacker]