Thanks for reporting this.
I've noted this behaviour, too. However in my case it goes back to available
most of the time and only sometimes it stays busy.

Currently, I still don't know what the problem could be.
Do you have a testcase to reproduce the problem?

Does that happen only with failing builds or also with different ones?

Maybe this helps:

java version "1.6.0_12"
Java(TM) SE Runtime Environment (build 1.6.0_12-b04)
Java HotSpot(TM) 64-Bit Server VM (build 11.2-b01, mixed mode)

Debian Lenny

Hudson ver. 1.321 (Debian package)

Plugins:

Hudson disk-usage plugin 0.9
Hudson TextFinder plugin 1.7
Emotional Hudson plugin 1.01
Hudson Task Scanner Plug-in 3.10
Hudson Mercurial plugin 1.16
Subversion Plugin 1.5
Hudson SSH Slaves plugin 0.7
Hudson Support Subscription Notification Plugin 1.0
Hudson Sonar Plugin1.1
Maven Integration plugin 1.321
Hudson Jabber notifier plugin 0.10
Hudson Dependency Analyzer Plugin 0.3
Hudson Warnings Plug-in 2.11

1 build processor

Only Jabber-ID and password are set in the preferences

This does happen with all builds (successful or failing).

Nevermind, I think I've found the problem. If you like, I can send you a
snapshot with the fix.

Code changed in hudson
User: : kutzi
Path:
trunk/hudson/plugins/jabber/src/main/java/hudson/plugins/jabber/im/AbstractIMConnection.java
http://fisheye4.cenqua.com/changelog/hudson/?cs=21209
Log:
[FIXED JENKINS-4337] busy status not switching back to available after build finishes

That would be nice

thomas@atunes.org

The problem is still present in the latest release (0.11).

hmpf, there seems to be no clean, reliable way to calculate the number of busy
executors with a RunListener.
I guess, I'll have to use brute-force i.e. polling for busy executors.

Could you please check if you have warnings like this in Hudson's log:
Didn't find executor for run xyz among the list of busy executors.

Code changed in hudson
User: : kutzi
Path:
trunk/hudson/plugins/jabber/src/main/java/hudson/plugins/jabber/im/HudsonIsBusyListener.java
http://fisheye4.cenqua.com/changelog/hudson/?cs=21639
Log:
JENKINS-4337: workaround for wrong executor count

There is no warning like this in the log.

Code changed in hudson
User: : kutzi
Path:
trunk/hudson/plugins/jabber/pom.xml
trunk/hudson/plugins/jabber/src/main/java/hudson/plugins/jabber/im/HudsonIsBusyListener.java
http://fisheye4.cenqua.com/changelog/hudson/?cs=21641
Log:
JENKINS-4337: workaround for wrong executor count

I still don't completely understand why it didn't work in your case.
Nevertheless it should be - finally - fixed in 1.0