I made a selection helper.. I know, it's ugly code so the others need
not comment 
In my case it was States and Cities though. If you take some time to understand the code you'll be able to use it as well.
Please see the following in order: 1) Rails Hacks and Other Stuff: helper: collection_for_select 2) Parked at Loopia
In the pastie, you'll see 1) state selection box in view --> this would be your "city" 2) encodeURI_selection helper --> just a helper that it uses 3) show_cities.rjs --> this would be your "show_companies.rjs" action 4) city_select.rhtml --> this would be your "company_select.rjs"
Remember -- your city is to my state, your company is to my city.. so
don't get confused 
