Windows 8 and LOB Apps

WinRT is for building Metro-style apps. If the app you're building doesn't fit the Metro style, then just build it as a traditional desktop application. Not all applications will fit the Metro style, and they don't have to.

Well, this is a bit old question now, but since situation is more clear nowadays (after release of Windows 8 and its tablet version - Windows RT with some new hardware by Microsoft) you may reconsider targeting WinRT instead of WPF for your new projects. This will allow your apps to run on tablets with Windows RT OS in addition to work on desktop in "Metro mode" and in the observable future (taking into account MS plans for unifying development for desktop and mobile by targeting project to WinRT) this also could simplify porting part of main desktop functionality of your financial app to its mobile counterpart version running on Windows Phone and allowing the user to do something useful even when they are on the go.

As for how much complex financial application could fit into "Metro" design and concept - you should have a look into several existing financial application in the Windows Store and compare their complexity with your possible requirements.

Another way to determine if WinRT will suites your desktop app is thinking about how complex will be breaking all functionality of your app into multiple relatively separated screens with input elements large enough for touch input by fingers - if that's looks doable, then you probable should go with WinRT.

But if the application is expected to show a large number of different tables simultaneously or, for example, should collect a lot of typed information by user (i.e. long descriptions, rich text formatting, lots of fields to fill etc.) then it probably will be usable in desktop mode only with hardware keyboard and large screen in front of user (i.e. on tablets/mobile it will be just painful for user to do all those things). In that case the WPF will suites just fine.

Suppose you stick with WPF as your target.

Then you could try to keep as much as possible of your code within the intersection of WinRT, Silverlight, and WPF. Maintain two (or three) projects and make sure that both (all) of them compile. Eventually you'll probably have to use something that's platform-specific to WPF. Simply stub it out for WinRT/Silverlight.

This gives you maximum flexibility no matter what Microsoft does with Windows 9, and allows you to keep track of how much of your code can be ported directly over. If they bring WinRT XAML to the desktop, you're covered. If they improve the Metro/desktop integration (a richer task switcher, say), you're also covered. If nothing happens, then you've only incurred a small development tax.

WPF will still be around after version 4.5. May not get many flashy new features that don't appear first in Silverlight or WinRT, but WPF is close to a mature technology anyway. Visual Studio, Expression Blend, and AutoCAD should be enough to secure its future support.