From a reader in the combox under another entry:
I propose that for Benedict’s new coat of arms, he should keep the keys but remove the mitre and pallium, replacing them with a white galero with 21 tassels.
I have not heard anything about this yet. This proposal, however, seems reasonable.
This is a rather new situation. In the case of the last abdication/resignation, Gregory XII went back to being a cardinal. In this case, Pope Benedict (I can still write that for a few more hours) will be (how strange to write this…) “Roman Pontiff Emeritus”. Thus, he can’t have the pallium. He changed the tiara to a miter, so it seems that should be removed. The galero, used but in white, could be a good compromise.