I'm rubbish at this stuff, so I post a reply then go away and try it only to find out it doesn't work ! Sorry.
You need to give your <OPTION> tags a VALUE attribute.
For my above example to work you need to do:
But, I think there is a better way:
<SELECT NAME=myselect onChange="document.formname.targetfield.value = this.value;">
I've tried this, and it works, I hope it is what you were after!