Melinda of Lion in the Sun suggested that instead of listing your email address on your invitation, you have a small card - it could be business card size - printed that instructs guests to RSVP on your wedding Web site or to a particular email address.
While I don't think there's anything wrong with having your guests reply via email, I do think you have to consider two things:
1 - If your guest list includes members of the "Greatest Generation" (age 70+) you have to consider that some of them may not have access to a computer let alone their own email account. (My grandmother is still amazed that we can shop for things online.) You may want to have a small amount of traditional RSVP cards and envelopes printed just for this set of guests.
2 - Your invitation sets the tone for your wedding. The more casual your invite, the more casual your guests will assume it will be and they will dress accordingly. If you expect everyone in formal attire then you should have your invite reflect the formality of the wedding. An email RSVP may reflect a more casual affair.
