A lot of people now skip the entire inner envelope concept when sending out invitations, which I think is fine- it saves trees, saves money, is less formal, etc. However, the nice thing about an inner envelope is that you usually write out the first name or even "pet name" of the person you are addressing. It adds such a personal touch. I think a really great way to combine the two would be to do something similar to the photo above- just add a good dose of familiarity and playfulness to the outer envelope by using just the first names! I think it would be perfect for an informal, whimsical event. Don't you? I hope someone takes me up on this idea soon for their invitations!