Just this morning, I upgraded my iPhone 4 GSM to 5.0 GM. I used SnowBreeze 2.8b8 to create a new IPSW and then iBooty 2.3 to boot tethered.
So far, everything has worked as expected. I installed MyWi 5.0 via Cydia (Purchased version - not cracked) along with a few other Cydia apps then rebooted as Cydia requested.
After this reboot (where I had to boot tethered) I opened MyWi and received the usual first install message to Reboot before MyWi can work. I've gotten this message many times before after initial install or upgrade.
I rebooted the iPhone and booted tethered again. When opening MyWi, though, it still has the message requesting reboot. I've tried it a few more times and even let the iPhone sit booting at the Apple logo (didn't run iBooty or go to DFU mode yet) and then restarted with iBooty but I still get the message in MyFi after that.
It is possible, MYWi 5.0 has not been updated yet to be compatible with IOS 5.0 GM . If tethering is important to you, then you should consider downgrading.