The problem here is that the "Enter" key may be used to submit the form if using some browsers. The first thing to address is how to disable the submission of the form when "Enter" is pressed, meaning you must trap the onSubmit event for the form and have a client-side script function called which will change focus to the next field and return "false" to the browser, i.e. as to not continue with the submission after completing the function.
The second thing is, how do you cause the form to be submitted using the submit button? You can't, because you're trapping onSubmit for your focus-switching function as described above. I suppose you could do something like having the form be submitted after passing through the last field on the form, etc., but it would be a hack however you do it.
I'd recommend communicating to your customer the hoops you'd have to jump through to get a shoddy implementation at best, and hopefully if he or she is reasonable, acceptance of the current technology at hand in return for the simplicity of design (and use... the tab key is easy to get used to) will make the most sense.
