I have the same problem with the UNO.
Running a G5 quatro powermac 10.4.11.
My mac recognizes the board as a modem as it should. And I can choose the right serial port (modem).
But if I start to upload it gets a sync or not responding problem.
I tried a few things and the only thing that sometimes works is holding the reset button starting the upload. And release it when it's compiled. Or sometimes just push the reset button when it's compiled.
If I try it 15 times it will work only ones. A bit frustrating :-/. So there got to be a work around for this problem.
I thought the UNO would be even easier to use. But that ain't the truth for me. Luckily a have a few other previous gen boards lying around.
Bubba5656 let us know if you got the board running on windows 7?