For example I go into bookstores, and then find that I can't buy a book because I can't look up any commentary about what other people thought of it.
Toss in geocoding, and you can start to leave virtual notes just about anywhere.
One interesting idea I read about was leaving notes for people at a public transit stop. You could end up with a little message board dedicated to a physical place. (Admittedly this would only happen somewhere other than the US where one finds middle class or better people taking public transit)
I should get some confidence and just build a wearable computer.
Oh yeah, and I wish I could build my own mobile phone as well. I'd like it to use SIP or H.323 (computer telephony) when I'm near a wireless network and switch to GSM/GPRS (ordinary mobile phones) when I'm in less civilized regions.