I’ve had people get a hold of me via the receptionist and a few more resourceful geeks got my direct line and called me directly. In one case I gave them some of my time, offered to set up an informational tour with an employee here who does that, I never heard back.
If someone walked it it would be purely the roll of the dice on whether I felt accommodating that day to take the time to review their work and give them a tour. I would, however, set the precedent right up front whether or not we are hiring. In this economy, if a position is not posted publicly, I wouldn’t bother walking in.
If you can do a good impression I would encourage you to try a Walken Walk-In, they might not hire you, but damn that would be funny.