Uno / Ethernet Shield / led in pin 13 does not work

Ok, I'm new.
I just received my first Uno and Ethernet shield.
I'm unsing the Arduino 1.0.5

Putting a single led into pin 13.

Running a sample like blinkwithoutdelay works fine.
Running a sample like webserver works fine.

But running a very simple modified webserver expecting to light up or down the led when receiving a web command does not work.
The led is continuously on (but not full power)
Also, it blinks while the html page is being transmitted....

I'm wondering if my ethernet shield has a problem or not !

Same symptom for pin 12 and 11. the led is always on but with a strange level of light

But with pin 9, 8, 3, 2 all works fine. I can switch on or off.

Any idea? Are the pin 13, 12, 11 not usable with Ethernet shield and Ethernet library?

Thanks

Well sorry, I'm still new.
I should have read more carefully the Ethernet shield specs.
Pin 13, 12, 11 seems used on the Uno.

Pin 13 is one of the SPI pins.