As per this.

Yes, if you mean the ones by http://www.velosolo.co.uk/
Does what it says on the tin, hasn't come loose yet, well made and not more expensive than 'normal' cogs.

looks like a very expensive way of going fixed

Works for me – a very inexpensive way of going fixed if you already have a 6-bolt disc hub 😉

Velosolo supposedly offer a lifetime replacement warranty too, just in case you ever wear it out.

Works for me too. I drilled one myself, a cheapo that came with an on one singlespeed kit.
Is a good way of going fixed because the chainline and spacing is easily shifted to suit different frames, and the XT hub is light and good quality.
I built mine onto an open pro rim, dishless with perfect chainline.

Mines for sale if anyones interested. (Whole bike)

Goldtec!

they seem really sensible to me – not sure why all fixed hubs aren't now made symmetrical & have a disc mount at either side (for 2 diff cogs, or cog & disc brake on the other side)

would stop WCA worrying (us all) about lockrings too

Actually, looking properly at hairy's link – what is that XT hub?
I assumed it was a standard (9sp) hub that had been reversed & the cog bolted on to the rotor mounts but it isn't, there's no freehub at the other end is there?

– is it a front hub with extra spacers, or what ?

It is a front with extra spacers and differnt axel, they sell a kit to do that too Here

But if you have an old road frame made of steel you can probably bend it to fit a regular front hub.