Potentially, but yeah, code issues. Windows Phone 7 is based off an elder Compact Embedded kernel than I believe NT kernel, which is what Windows is built on. Windows Phone 8 will have MAJOR code changes, like networking stacks and all, but I think Microsoft will still be using CE hybridized with NT or otherwise it would LITERALLY be a WINDOWS Phone.
Also, there's screen resolution issues. Metro apps in 8 are more designed for the 1366 x 768 resolution. Windows Phone as of now have screen resolution of I think 400 x 800. It might be tricky also since I don't know if an ARM processor's instruction set can work on an x86 processor. I think it might.
But then again, A LOT is possible with technology if you have the right tools and tech. I wouldn't be surprised to see an emulator for that coming along, it might come from Microsoft and it could theoretically be built into 8.
I doubt that though since Microsoft is promoting the Windows Store for Windows developers and the Windows Phone marketplace as something separate. Ideally though, a developer that made an app for Windows Phone would develop a similar app for Windows 8 to take advantage of screen estate. I've seen it happen, like with the USA Today apps for Windows 8 and Windows Phone. The content is the same, but the UI is totally different because the device's screen size is different and the UI needs to take advantage of that. The app is pretty cool for Windows Phone, as is the app for Windows 8 is even better I think. I would think it would be possible for developers to code for multiple platforms since Windows 8 is from what I hear easy to build on than before for metro apps, and Windows Phone is pretty good to develop on too.
Honestly though, I wouldn't fret too much about app selection since right now, Windows 8 doesn't have a huge marketplace, but two years ago, Windows Phone didn't either. Windows 8 is a different beast though since we already have an ok choice of apps in the Windows Store and 8 isn't even an official product yet. And considering Windows is a HUGE platform and the Windows Store is a first for Windows that will allow developers to get their software out there, I bet by the end of 2012, the number of apps will be MUCH higher than today.