Hi,
I have had the same idea and its implemented on the following website:
http://www.smart3.org/Kontakt_deutsch
Look at the source line 640. You will find several submit buttons. Only one is visible and can activate the form. The right submit button has a h2 in front which is outside of the view, but should be visible for users with screenreaders.
This methode is still experimental. Lets see if it works. If spam bots are clever enough to interpret css, it dosent works.