In order to use the Nuelectronics ethershield with another SPI device you need to change the 74HCT08D with a 74AHC125D. I've done it for one board using DIP version of the chip by cutting tracks and using thin wires. However, it can be done by directly replacing the surface mount chip as described here http://john.crouchley.com/blog/archives/662
, see second part of page.
You also need to use a different CS pin for both devices and update one of the libraries to use the alternative pin. I am successfully using this method in an RF12 to ethernet bridge where the RF12 module is receiving packets that are then being dumped into a UDP packet and sent out on the ethernet.
Using a SD card as a file store for html pages would be possible, but the current library is restricted to sending out html pages in a single packet. The size of the packet is dependant on your buffer size which in turn is limited to the memory size of the 168 or 328 chips. I've no plans at present to update the library to support this, but anyone else is free to have a go!