now that's interesting. you know, talking about these services you find out how people could/would use them. i myself would think of turning it off in your sewage example: i probably wouldn't have it switched on before i go there. my use case isn't "leave it on most of the time and only switch it off when you need to hide". my use case is "leave it off most of the time and only switch it on when you want to be located". so i'd probably have it switched off when at home or at the office because my friends know where and when i work, anyway, and i wouldn't want to signal "come and join me at my office".

i'd turn it on when i take a walk along the river danube, thinking: "hey, lovely day, wouldn't it be great to meet just anybody who's nearby right now and have a coffee?"
if nobody sees me and my walk is over, i'd go offline again.

very much the same as with IM: "online" means "ready for chat", anything else - incl. offline - means i might be online, but i'm sociophobic again.

right. reminds me of a friend of mine who had - well, mental problems, kind of a breakdown, left her family, bad story... anyway: after they'd started searching for her, in no time the police could tell exactly where she'd been, using mostly data from her cell phone, but also from cash machines etc.
in this particular case it was great because it helped finding her and giving her medical treatment as soon as possible (she's fine again now), but still... it was a little bit scaring to experience this, see the level of detail "they" have access to if they want. they certainly wouldn't need nokia friend view or google latitude for this.