Drop Down selection effects second drop down

I have a drop down that has all the towns in Connecticut as options.

I have a second drop down that lists 3 office locations. This field is admin only.

When a user selects their town in drop down 1, I need to conditionally select which office location they get assigned to in the admin only field.

I know it probably uses pre_submit and some switch function... and I've been shown how to do similar things here before, but I'm having a total brain freeze and just can't seem to nail it down.

Any help?

OK, so I got a great reply via my support request that straightened me out on my jquery... but I think my implementation may be wrong. I'm not sure where to go from here. And unfortunately, the client has me under the gun to finish up quickly, so if anyone here has a suggestion on how I screwed up using what Dave gave me, I'm open to help...
I put this in my theme's functions.php file:
http://pastie.org/5459903

Actually, there are a LOT more cases in the real thing, but this is simply a shorter version. I'm sure my pre_submission inclusion must be wrong, but I can't seem to figure out what I did wrong.

The form is on http://shoprecovery.us/trs-register/ the id is 15. The field the user changes is #25, and I need their selection to modify #24.

Nothing breaks, it just doesn't make the selection. I do notice that after the form is submitted, the page source shows the script prior to any html code at all... not sure if that's what's intended...