If your phone is rooted, you can copy your wallet from /data/data/de.schildbach.wallet/files/wallet-protobuf
If you can't root your phone, I would need to send you a special version that has the ability to copy your wallet to your sdcard. However, this is a quite dangerous feature, as all other apps you got on your phone can steal your wallet/private keys from the sdcard easily.
In any case, it would be a good idea to be able to encrypt the private keys: if your phone is rooted (and I would think that the intersection between people rooting their phones and people using bitcoins is not empty), another application is more likely to be able to access the wallet-protobuf content. And you could only allow backuping a wallet on the sdcard if the private keys are protected by a passphrase.