For streaming games services like OnLive, it should be possible to pack up and leave for a better offer on a competing service. Although that might be technically true, there is a certain level of lock-in that naturally occurs. As a result of playing games, you automatically accumulate things like save markers, achievements and friends, all of which add up to give the account itself inherent value or bragging rights. If you find a competitor with better rates, it is still a case of abandoning that old profile and building up a new one from scratch, possibly for the same games. It would take a highly rational person to leave the old scores behind, even for games that won't be played again, to get a better subscription elsewhere.
Mokalus of Borg
PS - Few people are that rational.
PPS - And probably fewer gamers.