Is there a recommendation for a hardware platform for developing/testing SideShow devices, or is the Emulator adequate?
I'm not really sure about sideshow in particular. But since sideshow uses the microframework I could give you my view on that. For just getting started the emulator is very good. You're going to use the emulator during development most of the time anyway so if you're just figuring things out it is fine to use just that. You can also tweak the emulator to emulate the specific hardware you want to use (add io ports etc.) so if you're not clear on what you want to do or what you need this is a good way to try stuff out.
If you know exactly what you want to build it might be a good idea to think about what hardware you're going to need as soon as possible. There are some developer boards out there for the microframework that you might use for sideshow. Or if you've got specific needs you might end up developing your own hardware and porting the microframework to it. I don't really know what you need so I can't recommend a specific hardware platform. There are some examples on the microsoft microframework site that you might want to look at. http://msdn.microsoft.com/en-us/embedded/bb267253.aspx look under the heading developer kits.
I hope this is usefull for you.