(I stumbled the answer to the question I intended to ask here, but I figured I'd finish the post for the community's benefit...)
Per Xamarin's (apparently incomplete) documentation on IPA support, I set up a project to build an ad-hoc/enterprise package (IPA) for my IOS iPad app in VS 2012. I selected the Ad-Hoc build configuration and then cleaned and built the project, but no IPA files were generated, unlike the Xamarin Studio screenshot in the documentation. Rather, a standard .exe file titled .exe is built by VS, but that's just the empty executable assembly for the C# console app project type that Xamarin uses, not the iPad app redistributable I was after. I even spent time playing with the iOS Bundle Signing project settings to ensure I had the right Identity and Provisioning Profile set...still with no luck.
Finally I found the Build Adhoc IPA... option on the BUILD menu in VS. It's this that kicks off the build of the IPA file, and I noticed on my Mac build server that it then wanted a keychain permission granted, presumably for signing the IPA file. I agreed, and saw in the Mac Server Log in VS that the archive build was apparently completed. I checked my VS project's \bin\iPhone\Ad-Hoc folder for the output, and...not there, again.
After rummaging around a bit more, I right-clicked on the project in Solution Explorer, and there in the context menu I saw a promising Show IPA File on Build Server command. I selected it, and Finder was fired up on my Mac, magically displaying the location where the build process had dumped my IPA file.
So there you have it, if you've been trying to follow the documented procedures for building an IPA in Visual Studio without success, I hope this helps. And Xamarin, please ensure that published documentation matches your release versions...or if you cover this procedure in a video or if I just missed it completely or something, please post correct links in this thread.