Google's new Latitude service looks to be half Big Brother tracking tool and half useful impromptu meet-up tool. However I can see a number of show-stopping problems which will stop this and other similar programs achieving their primary goal of real-time location availability.
For the iPhone there's the complete lack of background tasks, meaning that Maps will need to be your active application all the time you're distributing your location. Then, even for phones where this will happily run in the background, there's the issue of battery life, GPS sucks battery life at incredible rates, I would imagine that 90 minutes would see a full charge emptied on the phone, whilst even the mammoth battery on the HTC Advantage would struggle to deliver more than 2-3 hours of real-time GPS data - especially when you remember that this all has to be fed back to Google's servers over a live data connection.
In principle the Latitude Service has much going for it, not least the likely ability for Google to sell advertising based on your location. If you're prepared to put up with regular interruptions from your phone then the possibility are endless. Regular coffee shop visitor? How about a text from a coffeeshop you've never visited before with a free coffee voucher to try them out?Or from a shoe shop that has an offer on those shoes that you were looking at last week?
For Google its a license to print money and for small businesses its a way of competing with the internet and big name brands. Of course for users its a potential annoyance and intrusion, but also a potential benefit too.
And for some left field uses how about some of these scenarios? A warning system that an ex-partner is in the shop/restaurant/bar you're about to enter; notification that your boss has just entered the shop you're in, when you're supposed to be ill in bed; or what about a warning that you're going to be late for your next appointment and should call ahead as a courtesy?