GPRS/EDGE is a relayed connection and is not very useful for prolonged socket connections, tho in theory if your provider is not into ports blocking it should work, but if all you need is just to periodically connect to a specific device and pull some data from it there are plenty of ways to do it. Probably the simplest one would be to use an always-on deamon that will report to your servers IP of the each device whenever it changes - that way you'll always know how to get to any specific device. A simple custom bind solution could do all that automatically.

However, such an approach seems a bit overkill, and it won't work if the ports are blocked, and it's tedious work to log into each device separetly just to perform a simple GPS check. It would be wiser to write a simple web service, and a to write a script/app for N900 that will gather all the needed data, pack it, and send it to your service on predefined periods (simple cron would do the trick here). Yes, you won't have a direct access to the target device in that case, but do you really need to?

As for the uninstallable part, well, there are ways to make it difficult to uninstall, but if those delivery guys are a bit techie they'll find a way - they have root access after all. If those guys have no idea what xterm is, simple cron execution is more than enough to 'conceal' it. However, I wouldn't worry about it anyways - if they don't want to be tracked they can always disable internet connection on the device, or even turn off all the radios (airplane mode) and then all your efforts are in vain.