Good question, first off, sorry I've not been on the forum...
I want to remind everyone that we don't think our angels represent what actual angels might look like. We designed them based purely on our own asthetic tastes, and with the desire to depict them as polar opposite to the way most angels appear in hallmark cards etc. As to wings, while the bible does mention that angels fly, it is actually unclear as to whether angels have wings at all. Most references to wings revolve around Seraphim and Cherubim, two types of Angels said to have multimple sets of wings, and even then the wings function as sheilds to filter the Glory of God. So once again, we decided to use wings for asthetic reasons, and tried to give them a fresh look and feel that played off the traiditional notion of feathery wings.
Kira and Eli's journey to "earn" their wings is not biblical, but the concept of growing in spiritual maturity is. I remember as a kid thinking I was ready for everything and anything, and feeling so frustrated when my parents disagreed. In many ways, we never outgrow this boundry pushing, though we take it up with God as we get older. Kira and Eli represent the universal tendancy to want to do things in our own strength and on our own timetable, what they must learn is the same thing we must learn: God's timing is everything, and increased responibility must be earned, not given.
Thanks for the great question!
Courage and Godspeed,
Chris