My only thought (one track mind) just now is how to make a bluetooth application that ran a grid over mobile phones? It would grow dynamically across any installed clients it found. You could copy in anything, and it would have metadata for that stuff. If it found a phone capable of point to multipoint, it would send all to everything around it, and it would allow things to be informally shared perhaps in various levels of secrecy, but also with various levels of social comfort:
If you were in a public space you could set your phone to something like "contactable" or "surprise me!" and this would get you everything passing by on the other phones. Animations, texts, pictures, videos, ringtones and combinations of these would be the easiest things to transfer. You might choose to keep a record of what you got, and inspect it later or search. This could be good for social purposes.
If you were in a conference, you might wish to reveal more, such as work documents, and you'd set it to "show". There are already things that do this, but you have to buy them. Buzzer I think it's called. Not worth looking up even...
There are libraries and things that can be used for this, and I guess the first step is decking out a computer with a dongle and the SDK, or a bit harder but more comfortable in the long run, is doing this with linux. Then, once set up and running, Cobain is a comms API, and you can get media streaming stuff too.
At its simplest, Cobain acts as a simplified communications API; it saves the trouble1
of writing hundreds of lines of code required to discover the devices and their services,
handle the connections and the low-level sending and receiving of the packets
Another use could be an online world in a phone. Could be a stick man world, but where you can build and draw environments - with sound/sound effects etc, and sit in them, and where people could see yours. Would need in-phone authoring. The grid aspect would be that in logging in, again in a social scenario, you would see the world that other people had built - some chosen from "wizards", others built from a process of taking existing photos and faces grabbed automatically from the phone and getting the user to confirm/reassemble until they found what they liked. The idea would be to stimulate individual aesthetic expression. Again, there's a library that might help with this: The EPOC 3d engine at http://sourceforge.net/projects/symbian3d/ but no idea how good/supported/useful this will be. Might also be good to do this in SVG, as it's got good support on mobiles and could be easily transferred to a computer environment.
All of these depend on critical mass though, and the object at first is to get things to everyone, hence a lot of the initial effort should go to into the distribution of the applications themselves. - Apparently there is a man in London who distributes his animations over bluetooth on the bus. This doesn't need much - you just find bluetooth devices, and click send, one by one... A simple addition would be a "send to all", but how do people know what they are going to get, especially an executable? You need trust... One way is that this happens after you know the person anyway, so you'd need some kind of use case: a good reason to give them that application. Another would be to do online authentication via a download first of a checksum or a texting of one (which could pay for it) so that users would initially see this as an extension of existing web content. So this could be a plug-in for a social website.
Then there's bluetooth texting: Easy: just a program that sends texts, but sends them via bluetooth to everyone else, like a chatroom. Again, you should download and then just set to "chat with me!" to make you visible in the chat room. It might be worth implementing a buzz or a specific tone to identify you when you ask someone to join the chat. Why would you want to do this? Classrooms. (And this could be for aiding L&T not just subverting it!). Once you can have this text based back and forth communication - it's only one step further to add a storyline or an environment and make it a game, or add web links to connect with the online world, connect the texts to a real computer screen so they can be shared that way, or just whatever features emerge from it's use.
Because of the problems with multipoint etc, only very small networks can be built in a grid of phones/pdas (plus you have all the various competing OSs running on them... I go for Symbian in this, but I don't know stats...) I believe complete bluetooth grids are no bigger than 8 devices max, but you can probably get around this with partial networks - so each phone only connects to a manageable amount of other devices, and works intermittently with the others, so you have the illusion of a larger network, but actually it's many little networks each co-operating to form a bigger one.