It's been bothering me for a while that the ILS driver specification in the Wiki was outdated, incomplete and in some instances misleading. I've done a major overhaul of the page, bringing it (as far as I know) completely up to date:
My edits are based largely on the Voyager driver -- this is the driver that VuFind was originally developed with, and it has the most functionality. It's also the one that I'm currently able to test with, so I can confirm that, at least with Voyager, things work as described in the Wiki.
I did look at the other driver code as I wrote this, and I noted a few non-standard fields in return values that exist in certain drivers but don't seem to be used in the trunk code (probably related to local modifications at the institutions that contributed the drivers).
I would strongly encourage everyone who works with drivers to read over the spec, see if their driver of choice meets it, and correct any errors they notice. I recognize that there are some weaknesses in this spec, and I hope we can work together to address them. However, I think the first step is to have clear documentation and code that matches it -- this will make global driver changes a little less intimidating to undertake.