I've been involved with the Bitcoin project for a while, and there are steps in place to prevent this. Essentially, the network tries to maintain block generation at a rate of six blocks per hour (one every 10 minutes) by checking every 2856 blocks (nominally 2 weeks) if the rate was too high or too low. At that point, all nodes adjust their hash target such that it gets more or less difficult to generate blocks.
The net result is that more nodes or faster nodes can only really influence the market for 2856 blocks. There is discussion about reducing this number to lower that time, as well.
If you'd like to discuss this with some Bitcoin participants, drop by the IRC channel: #bitcoin-dev on Freenode. I'm Lachesis on IRC.
