Ok first I read the "can N900 be used as a satphone" and now this.
Sorry to point this out but emergency services can't enable your GPS and get your co-ordinats, hell most phones don't even have GPS, particularly in the US where both the networks and the phones are pretty backwards.
This is however mainly because they don't need to cell triangulation is very accurate (tens of metres), certainly in the UK all networks now have this technology and no doubt share it with emergency services and anybody else who can be bothered to ask. Equally they can share it with you, so you know where you are.
However there is another thread on this board somewhere which describes how to remotely access your phone over a data connection and report back GPS co-ordinates, but this relies on you setting the phone up correctly first and reporting its IP address to somewhere when it has a network connection.