Not sure if it applies specifically to neighborhoods, but many larger ISPs (notably NTL) end up with IP conflicts among users because they share IP addresses. Sometimes this happens only with inactive accounts, other times with active ones. Most of the people handling join requests have figured out which ISPs do that, so players shouldn't encounter many difficulties signing up. If rejected, they just need to respond to the mail and confirm that no one else is sharing their computer/LAN and will be approved.
Comcast, for the record, has never had these difficulties, and there have been no instances of active players sharing an IP address when they do not know each other or use the same computer/LAN. This seems to apply primarily to larger European ISPs.