This is exactly the kind of applications that would make use of my idea for the application, I already have the N800 and got it with the discount code for two ideas I was not able to release (too much work in the office :/), but I was going to try, one never knows.
My idea is to extend the gpsd daemon into the tablet (dunno if there's this service already, going to study a bit about it) by implementing a bunch of integrated services:

1) Location based events
The idea is to have a database of locations (user POIs) and to send custom DBUS messages depending on: being at, moving from, going forward a location.

2) stream of positions (what currently GPSd does, but "broadcasting it" locally.
One streaming endpoint, multiple receiving endpoints.

3) same as two, but broadcasting the stream *outside* the tablet, over the Internet to your authorized (and willing to receive it) friends.

On this service one can implement several interesting applications:

a) alarms based on location (warn me if I am near a place I'm interested in: post office, bank, ATM whatever)

b) logging (you have been near: this, this and this (based on your interests you've specified)

c) tourist mode: I'm in Rome and I am walking around the town centre, I don't want to go look at a map to see what's around me *everytime* I move. I want it to KNOW what I like and WARN me when I am at a walking distance from a store I might like. This is similar, or probably the same, as point a). Example: The tablet knows I like to drink coffee and I'm following my Italian friend to a place he thinks I like. While walking, in the neighborhood there's a coffee shop I would have never noticed. The tablet warns me about that and I might choose to let it drive me there.

d) I'm in Rome near the Colosseum, give me infos about it. Download the podguide of the Colosseum, video, audioguide.. whatever service I'm registered at.

e) nearby friends: I am walking down Fifth Ave in NY, my friend's walking next block. Warn me (warn him). Ok.. "let's go drink a coffee".
(the topic of this thread, is it?)

f) keep always updated with nearby free hotspots (FON, and the alike). When I look at the tablet, I want to know that 1 mile away there's a free hotspot.. ok, let's go there and look if I've got the e-mail I was waiting for.

g) you name it.

I want it a service, like the "presence" service, of the tablet, not as a standalone application I have to remember to start up before leaving.
I just want an option in the control panel (similar to the WLAN autoconnection) like "check every 5 mins if I'm walking outside the house and then activate the <whatever> mode".

I just wrote it down, I'll clean it to make it more readable, to look for overlapping features with GeoClue, MaemoPlazer etc.. it might be the framework they could base their service on...
Anyway, this is just the basic idea.

But what do you guys think of it ?