Ah sorry mate, I'm half thinking aloud!
To answer your question, at the moment I'm working towards some sort of UAV but it's all fairly new to me. When I wrote my dissertation I spent a lot of time researching pathfinding (of known coordinates, without any form of triangulation) and positioning through WiFi which has led onto this thinking.
My short-term aim is to send a robot out into a unknown area and to map it out and from there to navigate around that area. Obviously I'm aware that the AVR probably can't mange this level of mapping so I'll probably modularise and offload some of the processing onto an ARM or maybe use a nearby laptop for the heavy lifting.
I appreciate some of the kit is quite expensive, it does turn you off when you're just a hobbiest. Though I dont want to spend a fortune I dont mind "investing" in the kit if it pays off - assumign what I'm buying isn't a rip off 
After your comments I was researching IR beacons. That sounds quite interesting method in its own right and I'd be willing to give that a go. Do you know where you can pickup a module in the UK. Though saying that I presume it can't be to hard to build.
On a similar note, much of the beacon based position requires some form of triangulation or trilatertion. I've been looking around for details but if I'm honest most of the equations (even if the concept is simple) threw me. Searching for a library or some step by step intructions seem to leed nowhere - clearly I missed something in Math 101 
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Speaking of beacons, I presume you'd need to map the area of freedom to assign a beacon a static position for the idea to work. How do most people pull that off? Get the old tape measure out?
Society of Robots had some good information but most of their kit relied on encoders to estimate positioning.