It should work, I'm using ActiveX pretty extensively in my gadget without many problems(though security in Vista can be hard to handle).
Dan
If you want to go the IFrame route:
There's this article which includes sample code and pics.
And here's a sample WPF Gadget.
"Samples that access external files will not be able to access those files. This includes images referenced in the XAML and XAML files, even inside the gadget package.
This is not a security issue, but rather a problem with the way the “WPF/E” control resolves file paths and URLs. Unfortunately, there is no known workaround, such as placing the files on a remote server. Only XAML content embedded in the HTML files will load."
Here's another example for you that shows both WPF and WPF/E Gadgets