What does it say in your owner's manual about towing?  If it can be done,
there will be a section devoted to it.

Regarding why it would void the warranty, I would suspect it's a
transmission issue, but you really need to talk to a VW service
adviser....definitely not a salesman.  Other thing you can try is calling
Remco 1-800-228-2481  http://www.remcotowing.com/

-Bob

I am not sure what differences the new bugs have over the 2001's, but
my friend tows a 2001. The only problem he had was when he forgot to
take it out of reverse (stick shift) before he took off. Yes, he blew
up the engine and needed a whole new motor installed. It was out of
warranty.

Hank <~~~learns from others mistakes

We have towed an 03 Beetle with no problems.  I don't think VW supports it
officially, but we were never threatened with warranty issues.  Of course if
we'd had drivetrain issues during the warranty things might have been
different.