Chicken and egg problem. There are no (few) pre-paid data plans because there were no (few) contract-less phones. Now one end of the equation seems to be loosening up, the other can be expected to follow.
There are actually more pre-paid plans than post paid plans. Not as well advertised of course but of the top of my head:
Ptel,
Straight Talk,
Virgin,
Boost,
Revol,
Metro,
H20,
Page Plus