Large companies? Any company leasing office space with a badged security system (is, any modern office tower) would allow this.

Everywhere I've ever worked allowed 24 hour access. Never worked in an office tower though. Never lived on the West Coast.

Really not all companies even with a badged security allow this, in many places you need an authorization/pre-registration if you are going to come in late (say 2-3 hours past normal work day) or during weekends.

Well, clearly you're not in the valley, where people are expected to sleep under their desks and thank their employer for the privilege. ;)

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.

Nope and I would probably never want to work under those conditions :P

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.

Yep, $EMPLOYER requires a very good business reason for after hours access - safety / security are concerns.