Found an answer to an earlier Q in a Israel FAQ:
"Q: Can I jeopardize my citizenship by volunteering?
First of all, most countries couldn't care less. If you're from the States its
simple - don't tell them!
Seriously - the law regarding American citizenship works on the following
principle: In order oto lose your American citizenship, you must show the
desire to do so ("due cause"). I.e. - in the past, visiting a Communist country or
volunteering in a hostile army, etc...
Serving in the IDF even as a volunteer does not constitue immediate showing of
cause. In a case in Washington, someone's passport was withheld for doing
Machal. The judge ruled against the State Dept., and his passport was
returned. This is not a guarantee, rather a showing of the facts. No one to my
knowledge in the last 15 years has ever lost their American citizenship for
volunterring to serve longer in the army or for volunteering to serve at all. You
can, of course, ask the army to draft you..."
so there we go :)
I fully intend to volunteer to serve, and it's entirely likely they'll take me for 100
days...
Andy
Dawn Falcon