Pretty much any company is willing to be bought. It just boils down to what the company hopes to retain. For instance, Valve being the subject at hand has a few things going for it:
1) Its Half-Life franchise
2) Its general high-level of quality in all aspects
3) Steam
1 is a cash cow, for hopefully obvious reasons. 3 is an even bigger cash cow, which pulls Valve away from having to make all its money off of Half-Life. It also cements Valve into the industry for a potentially long time, as well as making things like the Blizzard Store and Impulse competitors instead of adopters, which may or may not be harmful to the whole idea that Steam was trying to create at this still early stage in its life.
2 could be a big hinge in any buy deal. 1 and 3 already would gaurantee a purchase price of high amounts. But 2 would probably mean that any purchase that were to happen would leave Valve largely intact and in control of itself. Basically, a company would buy it for two purposes: 1, to push its own products, and 2, to make money off the Valve name without having to do much of anything with Valve (since it is already capable of operating autonomously).
