Cannot work with arduino ethernet + usb2serial

Hello everybody, I am new to the Arduino world and meet some troubles with my new toys

Hardware
=> Arduino ethernet
=> Usb2Serial

Software
=> Ubuntu 11.10
=> Arduino-1.0 IDE + gcc-avr + avr-libc
Or
=> Windows XP or 7
=> Arduino-1.0

I try to compile and upload the blink.pde sketches and I get this message :
...
avrdude: Device signature = 0x000000
avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override
this check.

avrdude: Send: Q [51] [20]
avrdude: Recv: . [14]
avrdude: Recv: . [10]

avrdude done. Thank you.

I googled a little bit on the web and on this forum but don't see anything that could help me.
I bought 7 Arduino kit (arduino ethernet + usb2serail), same message for all.
I test with 2 USB cables, same message
I try to modify the wrong signature in the avrdude.conf, same message
I don't know where to add the -F option to avrdude ...

I test also under Windows XP and 7, the message is quite different :
...
avrdude stk500_getsync() not in sync resp=0x00
...

I am really lost and is anyone can help me I would really appreciate.

Thank you very much

I had the same error

You should try this just in case
http://arduino.cc/forum/index.php/topic,73748.0.html

It may have a communication problem , see what the loop back results gives you

Might even double check
http://www.arduino.cc/en/Guide/Troubleshooting#upload