I think I've got this working in the address-merge-field branch, but I need to do some more testing before I merge it in.

Note that MailChimp is very strict with addresses - you must supply a complete address or the sync will fail. In fact, all I wanted to do is sync a postcode (the UK's equivalent of a zip code), so I ended up setting up a custom "postcode" text field in MailChimp and populating that instead. This is why I didn't quite get the full Address functionality up and running.