Steps to reproduce:

1. Create a new NB platform maven project.
2. Select the deployment configuration.
3. Click on run. It will work. I'm not convinced it runs through JWS though.
4. Close the application.
5. Click on run again, and it will not work anymore, giving:

NetBeans: Executing 'mvn -Dnetbeans.execution=true -Pdeployment install nbm:run-platform'
NetBeans:      JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home
Scanning for projects...
------------------------------------------------------------------------
Building mavenproject1 - NetBeans Platform based application
   task-segment: [install, nbm:run-platform]
------------------------------------------------------------------------
[nbm:cluster]
Processing cluster 'platform'
Processing cluster 'foobar'
Created NetBeans module cluster(s) at /Users/simpatico/ws/mavenproject1/mavenproject1/application/target/foo
[nbm:standalone]
Building zip: /Users/simpatico/ws/mavenproject1/mavenproject1/application/target/application-1.0-SNAPSHOT.zip
[nbm:autoupdate]
Created NetBeans module cluster(s) at /Users/simpatico/ws/mavenproject1/mavenproject1/application/target/netbeans_site
Generated autoupdate site content at /Users/simpatico/ws/mavenproject1/mavenproject1/application/target/netbeans_site
[nbm:webstart]
[WARNING] Keystore related parameters not set, generating a default keystore.
------------------------------------------------------------------------
[ERROR]FATAL ERROR
------------------------------------------------------------------------
genkey returned: 1
------------------------------------------------------------------------
Trace
genkey returned: 1
        at org.apache.tools.ant.taskdefs.ExecTask.runExecute(ExecTask.java:636)
        at org.apache.tools.ant.taskdefs.ExecTask.runExec(ExecTask.java:662)
        at org.apache.tools.ant.taskdefs.ExecTask.execute(ExecTask.java:487)
        at org.apache.tools.ant.taskdefs.GenerateKey.execute(GenerateKey.java:416)
        at org.codehaus.mojo.nbm.CreateWebstartAppMojo.execute(CreateWebstartAppMojo.java:211)
        at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:483)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:678)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:540)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:519)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:371)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:332)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:181)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:356)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:137)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:362)
        at org.apache.maven.cli.compat.CompatibleMain.main(CompatibleMain.java:41)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
        at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
        at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
        at org.codehaus.classworlds.Launcher.main(Launcher.java:375)
------------------------------------------------------------------------
Total time: 17 seconds
Finished at: Thu Aug 05 19:04:27 AST 2010
Final Memory: 16M/81M
------------------------------------------------------------------------

I suspect the default configuration is wrong.

This is not the case with ant-based NB platform apps.