I agree - productxapp.com is probably best second choice after productx.com for an app name/domain combo, especially consumer apps. And make _sure_ that an existing productx.com service and domain are so different from yours in focus, there is no confusion for consumers who type productx.com by mistake, they know to keep looking for you. Also check US trademark and existing apps to make sure you can trademark it, else if you are successful you may find folks trying to clone variations of your name, or worse claiming your app name on different (android / windows mobile /...) app store if you just start with Apple, for example.
