I'm currently working with a client whose wedding will have multiple events surrounding the actual ceremony, for different subsets of guests. This is not atypical. Time is of the essence in the case of this bride, so Instead of coming up with multiple permutations of an RSVP card, she asked if I could create a form based 'e RSVP'. In other words, two different digital files that she could e-mail to guests to fill out, and then send back to her.
The response card requires a lot of information from invitees: accept/regrets for 3 events, meal preferences, song requests and also provides registry information. Since we weren't constrained by postcard or standard stationery measurements, we could make a very long form, complete with text entry fields and check boxes for the guests to fill out and then email back to the couple.
This is different from a standard e.m.papers printable wedding RSVP. Our editable form templates are for the wedding couple to fill out with their personal information and then print out. The forms are entirely invisible on the final product and the final product is a physical, printed card. I'm not offering these 'e RSVPs' as a product, but I have to say, it's a clever solution to both a time crunch and having a more complex set of events that require different response cards for different guests.