I haven't seen this app around yet, at least with this specific purpose. So I built it. It's called phone home if you search the market. It's the app with the default launcher icon
Phone Home [ALPHA] for Android
After installing the program on your phone, you must reboot it for it to take effect. On your first boot it will have you create a username/password. You can then log into my websites if you lose your phone and the phone will know that it should be updating its gps. You can then ask the website to tell you where your phone is and it will show the location on a google map.
This app uses very little battery since it doesn't actually turn the gps on. But that means it is only as up to do as whenever the gps was last turned on. There is no indication this app is running though. There's no launcher or anything. The only way someone would know is by using a task killer or something.
Be aware this is an alpha version. The functionality is there but the website isn't pretty and I don't have a dedicated domain yet. But give it a try if you want. Once I get it cleaned up a bit I'll probably release the source under the GPL or something.
Let me know what you think. This will always be free and ad free and will most likely be open source in the near future.