Interesting thought. The most customer friedly solution I could image would be a 'iTunes Match' like service for Apps. While seting up your Windows Phone, they could ask for your iTunes or Android Marketplace credentials and scan these accounts for past purchases of popular apps also available for Windows Phone and mark them as purchased in the Windows Phone App Marketplace.

Interesting suggestion. I wonder, would app developers even be willing to go with this idea themselves?

Sure,importing data is great, and should be possible, but why on earth would they hand out free copies of their software? I'm sure many would consider this complete madness