"[WP7] is a relatively closed system so you can't run native content, which means we can't really support it," explained Helgason. "We could, in theory, do what we've done with Flash, which is to rewrite the engine so it works inside a non-native environment. So it could be done, in theory, but it's very much in theory because that would require a huge amount of work and uncertain results."
With under 2 million WP7 units sold around May, sales don't really scream for Unity to go out of its way to ride the Redmond line. Meanwhile, Apple's recently launched iPhone 4S sold four million units in a weekend.