For those of you that have not seen it yet, SkyDemon has now put up a webpage with more details on the issue.
iOS 8.3 and External GPS Receivers
It seems that there are two protocol formats that an external GPS device can use to communicate with iOS, and the older of the two has now been disabled, or broken, or something.
So with certain newer devices you will have no issue whatsoever, and some older devices can be updated with new firmware so that they support the new protocol. If you can't upgrade your device to use the new protocol, you have a problem. Putting pressure on Apple and/or your device manufacturer may help.