How Can My Computer Know I'm At My Desk?

The problem is exactly as stated. I have my computer do certain alerting things, but unless I'm physically near, I don't it to do some of  'em. And it would be nice to be able to use it to enable certain security aspects, less "unlock when I show up" but rather "lock when I leave". But first, of course, we must have that mechanism.

So what are our choices?

• Motion Sensor - I like it. I used to have one in my den to control the light. It's neat. For work areas, I strongly suggest them for your lights, and have on this blog in the past. But you don't always have motion, especially when you've messed up line-of-sight or have a moment of stillness. So, it's a factor, but not the factor.
• Light Sensor - That won't tell me everything, but will tell us something, and that something is that the light switch is on at my desk. Not perfect, because often my coworkers turn the light on when I'm not yet there, but it's certainly better than nothing. 
• Location Sensing - My phone is the very definition of an Every Day Carry, and being a GPS-enabled Android which talks to Latitude, I can query Google to find out where I am. Except, where I am at work is in a subbasement, with no radio and no cellphone, so the location sensing defaults to a switch on the other side of campus, which hardly works. Maybe I should try it for "at home". 
• WiFi - If my phone is connected to my home network, my switch knows it and with a little coding, I can use it. Which is great, but when I'm at home, I often have WiFi off, and sometimes I use 4G (in the one corner of town where Sprint has 4G). Sometimes I don't though, which makes it a poor indicator at home. But it gets worse at work. My desktop is behind a firewall and my phone would be on the campus WiFi, so never the twain shall meet. I could write something that announces every 10 min or so that I'm on the campus network, should I ever get far enough into coding Android, but it isn't there yet.
• Bluetooth - This is the one I'm currently dreaming about. My dongle is Class 2, which means I have about 30 feet of range (and possibly less at the office). That's a nice range.

Without the addition of hardware I don't yet own, I'm thinking that the best solution is Bluetooth. I think that I could have fun making a dingus with Arduino that does motion sensing and light sensing (and maybe a thermometer) so I can get all sorts of data I can play with, but I can't play with that at work tomorrow.

Any thoughts I'm missing besides USB dongle and smart card you can think of?