A number of different groups, including some folks at the Silicon Valley Homebrew Robotics Club, are working on robots that can play soccer. The problem presents numerous challenges, and it will probably be a long time before we see robots capable of competing totally under their own power and control. But, for those of us that aren't willing to wait, and don't mind a few compromises, a company in Japan is marketing a "Capsule Robot Soccer Game - Mr. Soccer."
The robot players are controlled by humans (like you and I) via 27 MHz units - so, basically it's a re-engineered RC car approach. The robots have drive wheels, but are also equipped with legs and feet that stick out in front to kick the ball. They do a pretty good imitation of running down the field with their little legs pumping like crazy. The remote controls have left, right, forward, backward, and a neat kick mode I played with a set in the store, and had a lot of fun with it. The manufacturer's list price in Japan is 5,985 yen - (just under USD$60). They have a promotional video at http://www.b-brave.co.jp/sub2/MR.SOCCER-PV.WMV.
The robot soccer set is already in distribution in Japan, and can be purchased at the Takara "Garage" store in the Coredo building in Nihonbashi.
My initial idea for hacking it would be to leave the robots alone and hack the controls. One thought would be to have a camera mounted directly over the playing field with an interface into the computer to track the robots and the ball. The field and ball colors would probably have to be changed so that the computer can easily identify and follow them....