I ran into a similar upload problem recently on Debian Linux 5.0.6 and after perusing the Arduino troubleshooting page at http://www.arduino.cc/en/Guide/Troubleshooting#upload, I resolved it by pressing the reset button on the Arduino Uno directly after clicking the upload button on Arduino 0022. After that the uploads succeeded, so it looks like I only needed to do that once, but please read the upload troubleshooting link for possible tweaks to this procedure and other things you can try.
Since Mac OS X is Unix based, you should see logging in /var/log/messages similar to the following when the Arduino Uno is plugged in and recognized:
Jun 1 23:38:07 spectre kernel: [ 130.543595] usb 1-2: new full speed USB device using uhci_hcd and address 2
Jun 1 23:38:07 spectre kernel: [ 130.739353] usb 1-2: configuration #1 chosen from 1 choice
Jun 1 23:38:07 spectre kernel: [ 130.744052] usb 1-2: New USB device found, idVendor=2341, idProduct=0001
Jun 1 23:38:07 spectre kernel: [ 130.744059] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=220
Jun 1 23:38:07 spectre kernel: [ 130.744064] usb 1-2: Product: Arduino Uno
Jun 1 23:38:07 spectre kernel: [ 130.744068] usb 1-2: Manufacturer: Arduino (www.arduino.cc)
Jun 1 23:38:07 spectre kernel: [ 130.744072] usb 1-2: SerialNumber: 6493534323335171F111
Jun 1 23:38:07 spectre kernel: [ 130.913503] cdc_acm 1-2:1.0: ttyACM0: USB ACM device
Jun 1 23:38:07 spectre kernel: [ 130.914834] usbcore: registered new interface driver cdc_acm
Jun 1 23:38:07 spectre kernel: [ 130.914839] cdc_acm: v0.26:USB Abstract Control Model driver for USB modems and ISDN adapters
If there are problems with the device, you would likely see them there and may give you something to go by. Note that in this case the device is /dev/ttyACM0. I would be interested to see what it looks like from Mac OS X (presumably you are using "Snow Leopard" v10.6.7?). If you are using the 64-bit version, that may also cause interoperability issues.