Apple changed the behavior of DNS search list processing in Lion (OS X Lion: About search domains and name lookups).
This makes it differ from the behavior listed in RFC1536. If you need / prefer the RFC-compliant search path processing, add the -AlwaysAppendSearchDomains argument to the /usr/sbin/mDNSResponder process in /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
So:
<key>ProgramArguments</key> <array> <string>/usr/sbin/mDNSResponder</string> <string>-launchd</string> <string>-AlwaysAppendSearchDomains</string> <string>-NoMulticastAdvertisements</string> </array>