Silverlight's Out of Browser model works great on Mac, and has since its inception. Just develop the app and test it on Windows, it's basically guaranteed to act identically on Mac, unless you rely on a Windows-only feature (such as COM interop in Silverlight 4 Trusted Applications).
Edit: To further clarify my answer, nothing beyond the Silverlight Runtime install is needed to run OOB apps on a Mac. So if the user is able to run you app in-browser to do the install sequence, then they will be able to install the app OOB and use it.
This works with Silverlight 3+ only, of course, as that's when the feature was released. SO if the end-user has Silverlight 1 or 2 installed, they won't be able to install your app, but they could upgrade to the latest runtime to be able to do so (unless they're on a PPC Mac, as that only supports Silverlight 1. But most Macs these days are Intel anyway, so that's not really worth mentioning).
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