The web page describing the built in ArduinoISP sketch and its use with the UNO is out of date. I used a UNO yesterday to successfully load a boot loader on a 168 and a 644. Could this be changed now to reflect the current situation.
And what has changed so it now works?
As far as I know, the problem is the Optiboot bootloader, not the Uno itself. So maybe yours is an Uno with a different bootloader than original Optiboot?