The JRE we ship has a bug with InetAddress.isReachable() not being thread safe. Bug(among others) noted at http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6595834
Wes has verified that the latest update( 22) of java 1.6 doesn't contain this bug.
Steps to reproduce:
1.) Add InetAddress Ping service for host1
2.) Add InetAddress Ping service for host2
If one host goes down it may show as up since the icmp replies are not coded