In short - ranging will never work.
I've got a half-designed laser ranger module that I want to get selling on ebay, and have looked into the schematics of the n900.

There is no IR reciever of any sort that can resolve time accurately enough.

One billionth of a second is 30cm error.
The LED is probably not detectable by a sensitive detector when bouncing off something 10 meters away, so you need to have an error of maybe a hundred millionth of a second to make it useful at all.

The CPU does not really have the ability to easily time events like this. At least not without extra hardware.

Sonar can in principle work somewhat.

The IR emitter is designed in functionality - it did not come along 'for free'.
It's a seperate IR LED unrelated to anything else, and is connected via a transitor or two to a IO pin on the CPU.