Apple will introduce new versions of iOS and OS X at its annual developer's conference.
iOS 8.0.1 Causing No Service, Touch ID Issues on iPhone 6/6 Plus, Apple Support Recommends iTunes Restore
It appears that the issue is limited to users who have an iPhone 6 or an iPhone 6 Plus, but affected devices span several carriers, including AT&T, T-Mobile, Verizon, and Sprint in the United States. Rogers subscribers in Canada have reported issues, as have EE subscribers in the United Kingdom and Vodafone subscribers in Germany.
Restarting an iPhone 6 or 6 Plus does not fix cellular functionality, nor does attempting to use "Reset Network Settings," but some users have reported that restoring iOS 8.0.1 via iTunes or doing the initial install from iTunes fixes the cellular connection issues.
Apple support has also recommended restoring iOS 8.0.1 via iTunes to fix the problem, as noted on Twitter and the MacRumors forums.
@dmarsh1996 apple support saying restore via iTunes to 8.0.1 to fix issue— Andrew Roberts (@arob_87) September 24, 2014
Update: Apple has pulled iOS 8.0.1 from the Developer Center and it is also no longer available via an over-the-air download.
Update 2: Apple says that it is actively investigating reports of problems and has pulled iOS 8.0.1 in the meantime. The company also says that it will provide information as quickly as it can.
Update 3: It appears that option-clicking on an iOS 8 file in iTunes will allow users to fix the issue without a restore. iMore has outlined the process:
1. Download iOS 8 for iPhone 6 or iPhone 6 Plus
2. Option (Mac) or Control (Windows) click on Update.
3. Choose the iOS 8 firmware file you downloaded in step 1.
4. Let iTunes do its thing