I just sent out my wedding invitations and did an email RSVP, and so far things have been working smoothly. I have had a few non tech savvy people that have complained, but they knew how to get in contact with us if they didn't have email.
My big reason for doing so was because of all the postage and then people not even returning the RSVP card by mail (and I have to say that I have been known to be one of those people).
This was my wording:
The favour of a reply is requested before
(insert date here).
Please RSVP to
(insert email here).
LOL... I went simple. I didn't really go fancy with it because I couldn't really find the proper wording online. I hope this helps!!!