FULL PRODUCT VERSION :
java version "1.6.0_10-beta"
Java(TM) SE Runtime Environment (build 1.6.0_10-beta-b24)
Java HotSpot(TM) Client VM (build 11.0-b12, mixed mode, sharing)

Reproducible with other Java 6 verisons, e.g. Update 6 as well.

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
Java 5 cannot unpack .jar.pack.gz files created with Java 6. In my case I tested with Java 6 Update 10 [build 24] and Java 5 Update 15 with the command line utilities, but others have reproduced this with Java 6 Update 6 and Java 5 Update 11. Moreover the issue has been seen with Java 5 Web Start clients of a Java 6 server.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Use Java 6's pack200 utility to create a .jar.pack.gz file from a jar. Use Java 5's unpack200 to try to unpack it.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Pack200 files created in Java 6 should be readable in Java 5 -- at the very least with a "-target 1.5" option or some such.
ACTUAL -
It is apparently impossible to create pack200 files with Java 6 that can be used with Java 5!

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Corrupted pack file: magic/ver = CAFED00D/160.1 should be CAFED00D/150.7

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Create all pack200 files with Java 5 -- which means installing an additional JDK on one's server, having one's server app know this location, etc.

Overall this is a real issue specifically for Java 6 Update 10 as it really hurts the ease of Java client deployment!