One curious problem about developing on Windows Phone that has yet to be addressed (but will, eventually) is the ability for developers to have a place to beta-test their applications. The only way to do it is to give your XAP file to seemingly random people who have an unlocked device and hope they give you feedback. The issue though is it has to be unsigned and you're not really sure who you're givng your app too. Let alone the whole pay thing if you didn't make it trial-ware.

XNA-UK XAP tester service is evidently a place where you can "submit" your app to the site and they:

...will install your XAP on our test and production devices and check for the common Marketplace issues that you get knocked back on. We’ll document at some point what tests we do once we get a feel for how the service is operating.

The group's Terms and Conditions explicitly states that they won't distribute the work, alter ownership or do any other shenanigans. The service is free for now, but may change in the future, depending on how successful the program becomes. While perhaps this isn't the best solution, we like the idea of a tester community giving "bedroom coders" a chance to get feedback on their app before submission to the Marketplace. Granted, Microsoft will eventually offer a solution to this via the Marketplace, so developers can directly interact with users, but until then you can either try this method or post it in a forum in the open. We think the former idea is a little better.

Source: XNA UK User Group