Well for starters, yes almost anything that has a CPU can mine Bitcoins. People have actually compiled CGMiner to run on their android phones, and they have even gotten a few KH/s. Not really worth it, but interesting nonetheless.

When I say that computers will be useless for Bitcoin mining by summer, I mean that several vendors will be shipping ASICs by then. ASICs designed to mine Bitcoins have actually been in development for quite a while now, and will be the last step in the evolution of mining hardware. It's basically a device that you plug into a computer's USB port, and has a chip inside whose sole function is to mine Bitcoins. It can't do anything else except mine Bitcoins, but it does that one job exceptionally well.

People started mining Bitcoins years ago by using CPUs. Then they figured out how to mine Bitcoins 10x faster by using a video card (GPU). Now, ASICs will be 100x faster than a GPU, or 1000x faster than a CPU.

3 different manufacturers have offered pre-orders for ASIC hardware for Bitcoin mining: Butterfly Labs, BTCFPGA, and Avalon. Each device has their pros and cons, but they will all be leaps and bounds of any hardware that is out today.

So that's why I said that if you aren't even planning on buying anything until the summer, you should really just wait and see which of those 3 ends up being the best deal for you. Right now, it's a lot of speculation about which is going to be the most profitable, but in 6 months when you decide to order, you will have a lot more information to be able to make an informed decision.