So what does "Proposed Standard" mean? That might not sound very stable - "Proposed" - but that term does have special meaning in the IETF. The best way to explain it is that there are plenty of other protocols that you use every day that are "Proposed Standards", for example: WebDAV, TLS, LDAP, SMTP, SIP, and IMAP. Of course, if you want to know more about the IETF's standards process, they have it documented, in RFC 2026
Joe Gregorio talks (now at Google) about GData and how says: This is great news for us because the Google Data APIs are built on the Atom Publishing Protocol. The current Google Data APIs are based on early versions of the AtomPub specification and now that AtomPub is a Proposed Standard work will begin on getting all of our Google Data APIs compliant with RFC 5023.