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competitions/netwars
I know that SANS is currently running a NetWars. I know it's been going on a for a few months, so you may not be able to get in at this point.
Alternatively, if you're looking for a "responding to netwards" with some sort of forensic challenge, you might enjoy the Honeynet Forensic challenge.
DC3 is hosting a CTF style competition if that whets your appetite. http://www.dc3.mil/challenge/2010/