This may well be old news to some people, but today I discovered that you can prevent 'Find My iDevice' being switched off. I'm making the assumption that this extends to all devices, I only have an iPhone4.

The problem when it first became available was, if someone knew it existed, they could just turn it off. Until today I had no idea this had changed. 

If you don't know, here's how to do it (it's simple)

Go to: Settings>General>Restrictions 

Enter your passcode if it asks, if it doesn't ask for a passcode you will need to enable restrictions. 

Scroll down to Accounts, and select "Dont Allow Changes"

Done 

The minor side effect is that you can't alter any accounts for mail etc. unless you turn off the restriction. A small price to pay in my opinion.

It has to be pointed out that this does not stop someone just resetting the device, but I don't pin lock my phone, so i see this as less of an issue. After all, if they can use it they have no reason to reset it.

Update:

As has been pointed out below, you will need to restrict location services as well in order to find the device on the map. With location services off, you can still do everything except actually locate the device.

This is a wee bit more of a pain in the butt as it locks the location services as they are and prevents new apps using location services (for new apps you can unrestrict, allow and re-restrict) new apps do not seem to warn that they cant access the GPS. For some, this restriction may be too annoying.

For me, I'll just swear at the phone when new apps refuse find my location, then remember half an hour later and curse at myself.

Thanks guys, I've updated the post to reflect this.

Someone could simply turn the device off too, or put it in a lead box. If someone is that dedicated you don't have a hope, there will always be a way to disable it.

What this does do, is make it harder for the average dishonest person. Thieves aren't usually too smart when it comes to technology, but they do learn. Lets face it, an on off switch beside the words "Find My iPhone" doesn't take a rocket scientist to work out. 

With the number of people being caught by Find My iPhone, iPhones and iPods are often actively avoided. I know three people who have been burgled, and the iDevice has been left. All devices were in plain sight, one iPhone was even unplugged from a laptop and left behind.

This is a prime example of thieves learning. However, if they discover there's a simple switch to turn off the service, these high value devices once again become targets.

I would love to see Apple provide a way of locking down the service so it cant be disabled. When I ask to find it I don't care if it makes an international phone call.