How to instantly give people a pre-filled wallet to introduce them to Bitcoin?
I would suggest starting a wallet at My Wallet (blockchain.info). 


It's dead simple, all you have to provide is a password
You don't have to enter any personal info about the person that you want to give the wallet to.
As opposed to e.g. Instawallet your friend can change that password so that you no longer have access to the wallet.
It is "mobile friendly" so you can do it from your phone, as you requested
It has a sleek and professional look so it won't scare away new users