I dont really have a solution but:
Monday after i got the phone i had problems (it had been fine for 3 days) i removed all scans and managed to rescan one thumbprint
Since then ive only had minor issues (touch wood) i found last week when i was travelling in to, around and from London that it wouldnt read but it was more because of sweaty hands, handling tickets ect - i found that if i gave the sensor a quick wipe with the corner of my sleeve or a hoodie string it worked fine, since then i've not had issues - if my hands are sweaty it doesnt read first time but i wipe the sensor and it reads again no problem
On the day i first had problems i couldnt get it to read even when i was sure the sensor and my hands were clean and dry so whether i had a problem and now i dont or ive just managed to deal with the problem i dont know
I stopped in an apple store last week and spoke to them about it and they said so far only issues they had found was due to sweaty hands, the apple rep i spoke to over chat (and then on the phone) when i first had the issue said it was a software issue that had been reported
I dont know enough about the sensor technology to say whether they can improve its ability to get an accurate read in more day to day conditions (read sweaty hands) without compromising the accuracy of the scan itself but so far whats been reported in this thread is that people who have swapped handsets havent had the issue so its not something we can fix by replacing our phones, time will only tell if they can fix it with an ios patch but for right now, mine is working - not as amazingly well as it did when i first got it but i've not had reason to complain about it for almost a week
My only concern is that we all had problems after a few days of perfect performance and if its going to get worse over time then all of us having issues in the first 7 days of use doesnt bode well for the scanner lasting...