To have ethernet enabled for VICE on Linux, you must have libpcap and libnet installed and VICE must be run as "root" user (e.g. via sudo). See
this document for more information on this topic.
However, I've also built my VICE (on Ubuntu 8.04) from source, with the ethernet build option enabled. I've also installed libpcap and libnet and ran VICE as root user. Still, there's
no ethernet available in the Linux build (but it works with WinVICE). So even if you follow the instructions given above, ethernet functionality may still be absent in the Linux version of VICE. And I wouldn't wonder if it was disabled for security reasons, as running such an app as root user *is* a security risk.
Small hint: try to use the latest cc65 (e.g. 2.12.0) release along with the CVS version of Contiki 2.1. It may be a bit more complex to create a proper build environment that way, but compiling cc65 and checking out Contiki 2.x really is no big deal. And that way you always have the latest sourcecode of Contiki 2.x available through CVS.