Heads-up and bug report:
Working with a client who has a vendor using the leadCapture/save endpoint. Report was the vendor was suddenly seeing "gibberish" and a 302, and I correctly guessed they were seeing a gzip-compressed response. Since the request did not include Accept-Encoding: gzip, their client did not perform any decoding (not uncommon for client libraries to be this inflexible unless they were init'd to expect gzip).
Per RFC 2616 14.3, Marketo is allowed to do this if even if plaintext is possible, but RFC-SHOULD send plaintext if it can. Since this appears to have changed very recently I would recommend it be changed back.
(Yes, I hate server-side form posts as much as I always say I do. Doesn't mean I don't have to put up with them!)