[ramanujan]

Two ideas on your post regarding potential technical resolutions.

First, in a web environment, one is probably logging all API calls and their effects on the server side. With Windows, you couldn't do that as your users were hitting local code rather than an MS server. This makes deprecation of older APIs much easier, in part because you can see which of your functions a particular API user is calling and in which order. Done right, these backend metrics will make it easy to identify the most popular use cases as well as functions that aren't used much.

Additionally, there was an interesting HN post from a few days back on self-describing APIs[1]. If this catches on, it will also make it easier to deprecate old APIs, especially if the OPTIONS request returns JSON. With a little bit of reflection up front, client code can then determine the most up to date way to achieve the effect they are trying to get with this API call. This may slow down client code (requiring one or more redirects up front) but won't crash it. Apple's support of Rosetta for PPC shows that slow emulation is a good intermediate between outright breakage (a la Facebook) and infinite grandfathering (a la early MS).

[1] http://news.ycombinator.com/item?id=3841710