As of now, you can. Actually, apparently the big problem with the UNO is the auto reset - if you put about a 120 Ohm resistor from reset to 5v on the UNO then it'll work.Load the ISP sketch, disable the auto reset with the resistor, and then follow the normal directions.(I just burned 3 chips last week from my UNO - so I know this works).
June 2011 Haven't tried it, but perhaps the fastest way to get ArduinoISP working on the UNO is to disable the bootloader. Then when the board resets it goes right into ArduinoISP. Note: you will want some way to re-enable the bootloader.