The Venue Pro that runs WP7 will probably be a popular phone, but not necessarily because it runs WP7, but for its design. The Dell Venue Pro has a sliding design with a full portrait keyboard and a 4.1" screen that is slightly bent, which many people think it's amazing.
Dell is apparently going to launch an Android version of it as well, which of course it shouldn't really surprise us. If WP7 doesn't turn out to be very successful, at least they'll have the good old Android to sell millions of units like it does for every other manufacturer that has fully embraced Android.
Here's a video of Dell Venue Pro with WP7: