Wow.  That was a headache and a half.
Here's the setup:
Internet --> ISA --> Single Exchange 2003
RPC over HTTPS needs a SSL cert, and there were definitely some problems involving "target principal names" and then just a severe LACK of logging/diagnosis/error messages!  How are you supposed to fix problems if programs don't give you logs?!  Reminds me of Mac FTP clients like Transmit =)
I don't understand why logs are stashed away in GUIs.

Anyway, here's the useful information:
http://www.mcpmag.com/columns/article.asp?EditorialsID=758 (I've uploaded the PDF here: 0804mcp_rpchttp.pdf (496.24 KB)
and remember this:
Hold down CTRL and right click on the Outlook 2003 yellow icon in your tray, and say, "Connection Status"
There's the logging you need! =)
http://support.microsoft.com/default.aspx?scid=kb;en-us;833401 also helped.
isaserver.org wasn't very helpful at all - and neither were most of the sites I found via Google.
I got everything working by these two resources alone - so you should be able to too! =)