Ivo already pointed this out, but I want to point it out again: Boy Baukema writes a very nice entry regarding backwards compatibility on the ibuildings.nl corporate blog.

Backwards compatibility (BC) is a tricky thing to support, even when you strive hard to, as Boy puts it, "think hard about your API" prior to release. Somebody will always come along and point out ways it could have been done better or ways it could be improved. I've had to wrestle with these issues a ton since joining the Zend Framework team, and while it often feels like the wrong thing to do to tell somebody, "too little, too late" when they have genuinely good feedback for you, its often in the best interest of the many users already using a component.

I had the pleasure of meeting Boy last year when visiting the ibuildings.nl offices, and he's got a good head on his shoulders. He does a nice job outlining the issues and a number of approaches to BC; if you develop a project for public consumption, you should definitely head over and read what he has to say.