How do we know? A survey taken of developers asked what platforms they were developing for in 2010, and what platforms they were planning to start supporting in 2011. As it turns out, developers have harkened to Windows Phone 7 like a moth to flame, take a look:
What we have here is a jump from 6%, to 20%. In short, developers are expecting Windows Phone 7 to sell strongly with consumers, and thus warrant their development energies. If you wanted a vote of confidence in the platform, this is it.
Now as we know, sales numbers have been hard to nail down, but even our most conservative addition leads us to there being more than 300,000 WP7 handsets sold, and that number should grow to 500,000 over the next week or two. There should be plenty of space for developers, and with the Holiday season coming up, there is going to be a fat install base of new phones with very few apps to compete with, something that will make any hungry developer salivate at least a little.