Pre-purchases are still available on Steam, where you can even get 10% off both the standard game and the "Season Pass", which gives you the full game, the soundtrack and two upcoming DLCs. As an added bonus, they've included three new Team Fortress 2 items in the pre-purchase packages:
The Atomic Accolade - Skip past all the boring busywork of science and get right to the exciting part: the fabulous prizes! This prestigious ribbon certifies your bravery in the field of excellence.
The Quadwrangler - This quantum-collared lab coat includes a handy rubber pipe that siphons blood from your heart, where it is not needed, to your shooting arm, where it is!
The Professor's Pineapple - We don't know what's in it, but we found it in some rusty drums buried in a shallow grave in a forest, so let's face it, you'll probably get super powers if you drink it.
With mere hours remaining until the release of Quantum Conundrum, I think now's the right time to pre-purchase this inevitably fun game!