There are several ways you can do this. All require using scripts to bridge the gap from auto-complete text field to matching ID number. Here's a demo file that shows several such methods: http://www.4shared.com/file/plr_jbkk/EnhancedValueSelection.html
Feel free to post back here if you look over that file and have questions about it.
I looked at the solution in your demo file and it does what I want. However, I'm having a bit of trouble getting my head around the code, since it operates on the portal entries. I found your post entitled Tutorial: How to use an auto-complete drop down list when selecting records related by ID number...
The code there is much simpler and I would prefer to use that. Are there any gotcha's if I do use that code?
The demo file uses the same concept in a more sophisticated manner than the tutorial, but for the drop down lists, it is essentially the same method--just without creating new records in a portal when a value is selected.