There are many forums in the Apple and ATT web-world that have gobs of postings about 4.2 and 4.2.1 issues with all sort of items: GPS, wi-fi non-connectivity, 3G failures, etc. I understand that a "few" people always have problems after updates, but this 4.2 upgrade has far too many people with problems than I care to dismiss. Perhaps 4.2.1 is fine, but so is 4.1, for me. And I won't press-to-test just because there's a new fangled upgrade out there.
