I was using javafxpackager with java8b96 and it was working.
I upgraded to java8b100 and it stopped working.

When I use javafxpackager.exe now, I get:

C:\Program Files\Java\jdk1.8.0\bin>javafxpackager
The application has failed to start because its side-by-side configuration is in
correct. Please see the application event log or use the command-line sxstrace.e
xe tool for more detail.

C:\Program Files\Java\jdk1.8.0\bin>

When I look at the application event log (using the in-built windows 7 system administration tools), there are event records with red error icons in it with a source of SideBySide. On clicking on one of these error icons, I see the following message and accompanying data:

Activation context generation failed for "C:\PROGRA~1\JAVA\jdk1.8.0\bin\javafxpackager.exe".Error in manifest or policy file "C:\PROGRA~1\JAVA\jdk1.8.0\bin\javafxpackager.exe" on line 6. The value "FXVERSION" of attribute "version" in element "assemblyIdentity" is invalid.

Log Name: Application
Source: SideBySide
Event ID: 63
Level: Error
Task Category: None
Keywords: Classic
User: N/A

Google does reveal some MSDN articles that reference this SideBySide error, but I have not tried to troubleshoot using them as yet.

The weird thing is that this packaging process used to work fine on the same machine using Java8b96 and no longer works on Java8b100.

The administrative log error seems to indicate that the javafxpackager itself is not being packaged correctly, rather than something system specific on my machine.