If bugster resources (jar, etc.) is already in deployment cache which was created using a previous version of JRE such as 6u24, bugster can't be launched with the latest jre7 deploy pit build at:
http://rehudson.sfbay/hudsonws/deploypit/bundles/

Error from java console:

toolkit class not specified, checking settings...
----------------------------------------------------
c: clear console window
f: finalize objects on finalization queue
g: garbage collect
h: display this help message
m: print memory usage
o: trigger logging
p: reload proxy configuration
q: hide console
r: reload policy configuration
s: dump system and deployment properties
t: dump thread list
v: dump thread stack
0-5: set trace level to <n>
----------------------------------------------------
Using an AWT UIToolkit
Set toolkit to com.sun.deploy.uitoolkit.impl.awt.UIToolkitImpl
Java Web Start 1.7.0-ea
Using JRE version 1.7.0-ea-b255 Java HotSpot(TM) Client VM
User home directory = C:\Documents and Settings\ccheung

server running at port: 4152
#### Java Web Start Error:
#### Found unsigned entry in resource: http://bugster.central.sun.com/bugster/bugster.jar

Call stack from the error dialog:

com.sun.deploy.net.JARSigningException: Found unsigned entry in resource: http://bugster.central.sun.com/bugster/bugster.jar
   at com.sun.javaws.security.SigningInfo.getCommonCodeSignersForJar(Unknown Source)
   at com.sun.javaws.security.SigningInfo.check(Unknown Source)
   at com.sun.javaws.LaunchDownload.checkSignedResourcesHelper(Unknown Source)
   at com.sun.javaws.LaunchDownload.checkSignedResources(Unknown Source)
   at com.sun.javaws.Launcher.prepareResources(Unknown Source)
   at com.sun.javaws.Launcher.prepareAllResources(Unknown Source)
   at com.sun.javaws.Launcher.prepareToLaunch(Unknown Source)
   at com.sun.javaws.Launcher.prepareToLaunch(Unknown Source)
   at com.sun.javaws.Launcher.launch(Unknown Source)
   at com.sun.javaws.Main.launchApp(Unknown Source)
   at com.sun.javaws.Main.continueInSecureThread(Unknown Source)
   at com.sun.javaws.Main$1.run(Unknown Source)
   at java.lang.Thread.run(Unknown Source)

I think the following steps can recreate the problem:
- install JRE 6u24
- run bugster http://bugster.sfbay.sun.com/bugster.jnlp
- exit bugster
- install JRE 7 pit build
- run bugster again