I've been doing some further experimentation with autodiscovery. First of all, while Apple supports autodiscovery for Exchange 2007 servers, they officially do not support it for POP3/IMAP servers. I've filed a bug report for a feature request, asking them to fix that.
I've also filed a bug report with Mozilla, but they seem opposed to the idea of adding support to Thunderbird. In some ways, I can understand why - they don't want Microsoft's autodiscovery protocol to become popular, because the protocol sucks; they want to design a better system and get everybody to support that instead. Of course this better system doesn't actually exist yet, and support for Microsoft's protocol could be added now, but if nobody wants to do it, it's not going to happen.
Microsoft has this great tool for troubleshooting autodiscovery issues: the Microsoft Exchange Server Remote Connectivity Analyzer. Unfortunately I've found not just one but four bugs in their autodiscovery implementation so far; clearly their test site doesn't share any actual code with Outlook, they just wrote a client implementation from scratch according to their understanding of the spec, and... they didn't do a very good job. I've reported the following bugs to Microsoft:
/Autodiscover/Autodiscover.xml, which doesn't matter on IIS but breaks if you're using a server that enforces case sensitivity. Either the spec should say that these paths must be case-insensitive, or the implementation should match the spec and use lower-case paths.
<Action>redirectUrl</Action>or use a standard HTTP redirect, but the test site thinks getting an XML file with HTTP status 200 is an error.
I wonder if they'll bother to fix these...