Okay, I didn't look this up to see if someone else is doing it, but I was thinking about Moblogging and the idea of real world annotations - where you can mark a space using GPS and then someone else can "see" what's happening at that space when they run into it with their cool high-tech mobile phone.
But the thing is that GPS or positioning enabled phones are several years away from being common, so what can we do to enable this now? Well you know how you go into a museum and you can rent those audio guides where you punch in the number of the painting to get the details? Well, what I'm thinking of is something similar - along with a Warchalking-type campaign to mark spots where someone has left their thoughts.
I mean, the first thing I thought of was "Hello My Name Is..." type stickers where you could paste them around town, etc. I could see a system where it's like a community blog and the number is the permalink. Someone decides they want to mark a bus stop with some info: "The bus always arrives 20 minutes late on Fridays because of heavy airport traffic". So they use WAP - OR even more easily - send off an SMS or MMS review (the MMS could have a photo for example). The server then responds with a number, which the person writes on a sticker and slaps it somewhere unobtrusive, but obvious to find. The next person that comes along can use their mobile phone to call up that comment - and here's the fun thing - annotate the original or leave comments. Like you would at this blog. (I guess one is like a Wiki and the other's like a blog or an open forum).
All this could done via SMS or MMS messages - which would be expensive for the server side - look at Locust's problems. Or via a WAP browser which is low-hanging fruit - I could whip this up in a day, though it's not as easy to use. Using this system you could perfect what works and doesn't work and then when GPS arrives, easily exchange the numbering system for a coordinate system...
Anyways, that's my brainstorm for the evening.