The Do Not Disturb button on phones may have been the first modern presence indicator. Reinhold Henke says it started...  

"In March of 1967 the Northern Electric Company (more recently known as Nortel) was granted a patent (3,321,580, pdf) that claims a privacy feature button on the station set that would allow the PBX stored program control system to reroute the call away from the called station. This common feature on the feature set station phones came to be known as the Do Not Disturb Button (DND). This feature became so popular that soon after companies like Mitel, Rolm, Siemens, etc. followed the call of the Northern Electric Company..."

The DND button was a signal (meaning uncertain but determined by convention) used by a telephone switch to inform the routing of incoming calls.

Six Observations:

1. The purpose of DND was decision support. In this case, routing an incoming call.
2. The original signal was continuous. Hey, it was all electronic, pre-digital.  
3. The signal was tied to a specific desk phone. Not to a person. Or a group of people.
4. The signal stopped at the deciding engine. The incoming caller didn't see the signal and had no influence over the call routing decision.
5. Logic not in phone. Oooh, a clue!
6. The signal was binary. Just "take calls at this phone" or "don't take calls at this phone."   

Six Lessons:

1. The purpose of presence is still decision support. We can support more decisions than call routing.
2. Presence signals need to be "fresh," not continuous. Freshness just means we can trust the signal enough for it to be useful in our decisions.  
3. Presence needs device independence. Many of us are no longer tied to one phone. Or even one computer.  
4. Presence data is useful even if you don't share it with callers. The logic for call routing ran separately from the source of the DND signal. Indirection, abstraction, and logic are friends of privacy and convenience.  
5. Separating sensors/signals from logic engine(s) works. Decentralization offers value. More on this soon.
6. Presence values can be rich. We can be "back in 5 minutes" or "just take calls from my family.". Presence can signal more now, with all sorts of data to inform many kinds of decisions.  

Images from the patent:

fwd/dnd button by clarkk.