Update: Reports suggest that Microsoft has plugged the Insider app. Using Fiddler to spoof Windows 10 on your unsupported device will no longer work. Although this was only one method to install Windows 10 unofficially, it was a popular one for those without an SD card.
If you're running the Windows 10 preview for phones on your device, an update for the Windows Insider app is waiting for you bringing it up to version 22.214.171.124. There's no telling what the update does, but it would be a good idea to go ahead and pull it down.
Feel free to hit up the source link below to grab the update. In the meantime, be sure to head on over to our forums and chime in on your experience with the Windows 10 preview for phones thus far.
Thanks to everyone for the tips!