For the past couple of years I've been running a small Windows-based web-hosting operation with the AD domain matching the company's public domain name, in accordance with my interpretation of Microsoft's guidelines of domain naming (e.g. "hostingcompany.com")
Obviously this caused problems: the lack of a split-level DNS (I foolishly used the AD DNS servers as the public DNS servers too) means that anyone outside the organisation can resolve addresses of hosts (the servers all have only one IP address each, which is public, and consequently marked as being on a "Domain" network with the relaxed firewall rules that go with it).
The system isn't sustainable and when we rebuild later this year (along with having the servers on a private network in addition to the public one) we'll be using split-level DNS, but I'm thinking of using ".local" as the TLD of the domain rather than "internal.hostingcompany.com". My reasoning is because we'll be offering white-label and reseller services, and our resellers wouldn't be happy with people seeing our company name as they go about business, which is why I was thinking of using the generic "host.local" domain name (it also has the added advantage of being nice and short), but I have reservations about using ".local" because of a lack of RFC compliance and issues with mDNS (even though it isn't used in my organisation). I'm just disappointed that Microsoft didn't petition ".internal" or something similar specifically for this use.
I've been told I should put an actual question in here, so in summary:
"What should the FQDN of a Windows-based hosting company's internal Active Directory domain be?"