And that is done by software only ?
i found this:
One little catch is with Ethernet shields based on the Wiznet W5100 chipset, which includes the official Ethernet shield and many third-party shields. The W5100 has a design flaw that causes it to maintain control of the SPI bus even when another device has been selected, making it very tricky to combine the official Ethernet shield with other shields that use SPI. The Freetronics Ethernet Shield uses the W5100 chipset but includes a fix in hardware so it doesn't have that problem and will happily share the bus with other SPI devices.
source: Stacking Arduino Shields | Freetronics
So maybe there's a problem with ethernet shield, even if i can use another pin for CS (on the nrf24l01 chip), the ethernet shield wont let me use it :s