A chemical toilet is also a wonderful thing.
Agreed, certainly at any company that has strong security policies (I'd imagine telecom, for example, is a lot more strict in this regard), after hours access might be an issue.
Most office buildings I've seen have restrictions, the office tower i work at in London (Reagent's Park) requires you to enter and sign in with security from 7pm till 7am and if you want to enter during the weekend you need to register explicitly for the day and tie and it has to be approved.
And this is pretty much the case for most shared offices / office towers I've worked in, if you own the campus and want to keep the same level of security staffing during the night as you would during the day I would think that 24/7 access wouldn't be an issue to facilitate in all other cases they rather that you wouldn't be hanging around.
Google allows 24 hour access, unless that's changed recently.