We've recently rec'd 3 orders from registered customers that have come thro' as Guest and do not have an email address.
The users have said that they had added stuff to their basket & some time later checked out. At no point were they required to login.
All 3 were paid for using Paypal Express - we can see the paypal user's email address
We're running CE 188.8.131.52 (patched)