Khuutra has pretty much nailed the issue with some excellent posts, however, I'd like to point out that the size of the PC second hand market doesn't really matter, as this scheme (and others before it) are effectively a trial run before they're brought to consoles. In essence they're seeing what the market can bear and what people will put up with before going for the bigger prize of console second hand sales.
That's why it's imperative that console owners also make a stand, otherwise you'll be having to activate your games and tie them to your PSN/ Live account before long. As Galaki says, some responses in this thread are unbelievable and certain people need to leave aside their petty tribalism and get much more clued up. Comments about MMOs or along the lines of 'well, everyone has internet access these days' are ignorant at best.
These companies are not your friends, they're international mega-corps who only exist to make money for their shareholders. Trying to defend their anti-consumer actions as if you were taking sides in a football game is quite ridiculous.
Equally, PC owners in general and the ones protesting against DRM in particular are not your enemies. Quite the contrary, if companies can be persuaded to drop these schemes due to consumer backlash, they'll be doing you a favour. Look at how EA have responded to the Spore/ Mass Effect debacle- they've dropped the stick of SecuROM activations, instead they have introduced the carrot of rewarding the purchaser by giving them extra in-game items, items which are not available if the game is bought second hand. This wouldn't have come about if everyone had just rolled over and accepted their DRM. Now it's how it should be- give people a reason to buy your game and they will do; make your game 'worse' than the pirated offering and that's what they'll gravitate to.
In the end, we're basically all consumers and we should be sticking together against companies trying to erode what we've traditionally accepted, otherwise we'll all end up shafted.