If you recently upgraded your previously developer unlocked Windows Phone to Windows Phone 7.5 (aka Windows Phone OS 7.1 aka Mango) you likely have found out that you need to re-unlock your phone. The process is pretty simple for most using Windows Phone Developer Registration tool that gets installed with the SDK.
Some people however, including myself, have run into an issue where deployment of an app to the now “unlocked” device still kept failing with the error message “failed to connect to device as it is developer locked”. This sure was a head-scratcher.
You can verify that this is the issue that you may be seeing by going to your account profile in AppHub and clicking on the “devices” tab. If your device really was successfully registered, it should show up in the list. If it is there, then your issue may be simply that the dev account for the device expired. Simply remove the device from the list and try registering gain.
If your device isn’t there, then you’re hitting the same issue that I did. It turns out that when the device is re-registered, it is re-associated with the account that it was originally registered with, not the Windows Live ID account that you specified when you register the phone. You can verify that this is the case by trying to register your phone with a Windows Live ID that you know not to be associated with AppHub. If registration “succeeds” you’re more than likely hitting this exact issue.
The solution to this is to have the device removed from the devices list of the original account before re-registering. I was lucky enough to get in touch with the person who had originally registered my phone and my problem was solved. I hope yours will be too.