The Selenium Plugin tests the Reachablility of a client machine using the Method
hudson.model.Computer#getHostname().

While the algorithm is pretty cool, in our network setup (virtual windows server
on a xen cluster) it fails because of a too small timeout in the
InetAddress.isReachable() - Method. Due to that, the Selenium Plugin does not
start any Remote Controls quitting with "Unable to determine the host name.
Skipping Selenium execution."

An increased (I'd suggest 3000ms) value in the InetAddress.isReachable() Method
makes the plugin work in our network setup.

hudson.model.Computer#getHostname()

Line 562 (old): if(!ia.isReachable(500)) {
Line 562 (new): if(!ia.isReachable(3000)) {

Could you increase the timeout?