Multi-target rapid prototyping using Microsoft technologies?

I want rapid prototyping: Quickly define screens, links, buttons etc. I want to easily interact with sample data. I want to see things "moving".

An example would be a mail account: I would like to say that a message has a sender, date, subject and content, then say that a "message" screen displays a list of message headers, and clicking on a message header opens a "view message" screen where a "reply" button leads to a "create reply" screen.

You know - Gmail :)

There are two catches:

1. I want multi-platform targeting: At this stage I'm not yet sure which parts of the project would run on the web, on desktop clients, or maybe on tablets and smart devices (Ok, Ok: iPad/iPod/iPhone).
2. I know and like Microsoft technologies: C# (or VB), ASP, Visual Studio etc. So I wouldn't like to switch to Rubi, PHP etc.

I thought Sketchflow was the answer, but this is Silverlight which is not portable.

I believe this need is not mine alone, so probably a solution exists out there!

Any suggestions?

Like you say, it's a difficult choice. Either you stuck with MS Technologies and the answer is Sketchflow (then Blend) and Silverlight which make development very easy, but that are not "standard" : works well on Windows, MacOS and quite well in Linux with MoonLight. Silverlight should work well with Windows Phone 7 but won't work with Apple Iphone/Ipads.

Either you can do it in HTML5, which will be standard, compatible with the most recent browsers (forget about IE 7, Firefox 3 and co) but then the development will not be as easy.

I don't know your app's complexity but i would start doing silverlight + webservices and a Iphone/Ipad app later (using the same webservices via json).

For information, the silverlight penetration rate is 55-60%.