Ok, for Mega you need to change the pin 10 to pin 53.
I did the same thing today on my Freetronics Ether-Mega, works great.
Also, if you are using the Arduino ver prior to ver 1.0, say 023 you may wish to upgrade to 1.0.1.
I asked that question as I was wondering if you had copied code from the Arduino site and pasted it into your older version IDE.
Change your pinMode to be 53 instead of 10, and hopefully you'll have it working nice.
Good luck
rockwallaby . .