I'm filing this bug on behalf of an IRC user because it has confused me as well in the past, and would make an excellent "paper cut" to fix. If you are you trying to debug a slave issue and look at the log, the last thing you will see is likely something like:

<===[HUDSON REMOTING CAPACITY]===>channel started
Slave.jar version: 1.343
This is a Unix slave
Copied maven-agent.jar
Copied maven-interceptor.jar
Copied maven2.1-interceptor.jar

with the AJAX thing still spinning (I assume because the slave is still running and as such still logging). However it is unclear that at this point the slave is successfully connected and functional. It would be really helpful to new Hudson users unsure if their setup is working to display a message in the log to the effect of "Slave successfully connected, will continue to log any errors" at this point!