If you've checked your Windows Phone 7.5 device for the 7.8 update with no success, you may want to try to force the process, just as we did with other Windows Phone updates like Mango.
This is a little different than downloading and installing Windows Phone 7.8 manually, which we reported on earlier. With this method you're basically tricking the Microsoft servers to recognize your Windows Phone and initiating the update early. Think of it as cutting in line (and Microsoft is okay with this, no worries).
The other benefit with this method (versus the CAB one) is it means you are getting the official carrier/OEM update that usually includes the firmware too. If using just CABs you are updating the OS but not any device specific software. That's not a bad thing but it's really not ideal.