This is a rather simple idea. Just start with a very accurate GPS device (likely requiring much better GPS technology than we have now, but in 10 years, likely it will be quite possible). This can have voice software, and just a couple of sleek buttons (Mac version), or a big thumb keyboard and stylus
and screen, etc. (e.g. integrated into a blackberry).
For more directionally important applications, the recently available Wii controller is a superb tool for multiple angle and position sensing at a very affordable price. This is a fine proof of concept for our ability to nail down everything hardware-wise here on the cheap, except the GPS sensitivity
What it does is this:
1) You move the device anywhere in space and push a button to take down the coordinates OR punch in known coordinates someplace in space where you are not. (I can type in my girlfriend's room desk or pillow while going to school in a different state)
2) You record a message, or type a message, or draw a diagram/picture, whatever.
3) You set an optional limited lifetime, password, viewing permissions, etc. for your note. They could just be public notes, too.
Somebody else with a similar device can then scan an area for notes left by others (can be set to automatic), and then discover where the note is tied to and what that note is, if they come in time to read it/are on your friends list/know the password/etc.
Basically, this is like a pad of sticky notes and a pen, except it can handle 3-d pictures, sound and video clips, has time limits, notes can be left from 1,000 miles away, and they can be suspended in mid air.
Additional options:
-The ability to manage complex networks of notes remotely from, say, a laptop, with weird permissions, possibly even tied into java programs, etc. For instance, if you are coordinating a complicated trivia scavenger hunt, or want to leave auto-updating time-sensitive data about a lab specimen for coworkers. Pehaps if you wish to use the system for artwork with writing that flies around in patterns in a gallery. Or if you want to have a visible marker (using the HUD idea below, for instance) to pace out a runner's time trial.
-Taking the above point further, make a standardized programming interface that allows random products to easily tap into this technology, even if the manufacturers have little competance in programming/electronics and are unwilling to pay for that expertise. This could replace those laser levels on power tools, it could allow an easy platform for all kinds of wireless baby monitoring/etc. that requires too many clunky devices otherwise, it can aid in astronomy by feeding the input you should be seeing into a telescope that allows you to plug in your everyday GPS directional device, with notes of interest from local hobbyists and professional astronomers to boot, and virtually no additional cost aside from a wire jack and an lcd screen. Or one surprisingly simple, yet stunningly powerful application of this would be merely having a pen with a temporary switch that alows you to essentially write directly on air around you.
-A HUD that actually overlays notes in appropriate relation to your visual field, and adjusts for font size based on how close you are.
-Little reference chips you can buy that allow you to use this system on moving surfaces. For instance, I could put a reference chip somewhere in a boat or a car, and any notes left in the vehicle would move appropriately with the vehicle when associated with the "My car!" reference point. Or backpacks, or a person (in their glasses? implants?) This could allow a number of more practical benefits, like IDing a stolen car despite extensive gutting, or carrying medical information, or allowing sensitive information to reach exactly those in a position to act on it correctly (one layer of encoding...). It could adjust for earthquakes, or the wind making the top of a skyscraper sway by a meter every minute.