Got an Airbike SL2 myself, though only handlebar mounted. It’s a good light, better than the P7 torches? Well it’s got a convenient battery pack that lasts a full 3 hours on full beam, so its more convenient by some margin.
That said, I spoke to Planet X and they don’t have any helmet mounts for it, or any cable extensions (the current cable is too short to mount he unit on your head with the battery in your camelbak), and don’t plan to do so it seems. Could of course make your own though!
Regarding the P7 torches, the one that z1ppy pointed out is not a "C Bin" meaning it’s not the latest and prightest P7, that said it’s probably the cheapest and 5 modes means you can alter its brightness a bit. Having a P7 light (My airbike SL2) on the bars, I wanted something with a bit more of a spot beam on my lid and went for one of these:
http://www.dealextreme.com/details.dx/sku.16240
It’s the same LED as in the Exposure Joystick (and the same as Hope use in their Vision 1 light) but a fraction of the cost. It adds a useful spot when you’re riding downhill anyway. The fact it will last over 2.5 hours on a full charged battery is also a bonus too, as it minimises the need to swap a battery whilst out on a ride.
That said, I’ve since noticed a much smaller/lighter unit on dealextreme:
http://www.dealextreme.com/details.dx/sku.13095
It uses the older Q5 Led rather than a new R2, but it’s only 20 Lumens less output. But the unit is about half of the weight of the R2 torch I bought (which in itself is not heavy), so you could use 2 of them on your helmet, for the equivalent output of a Hope Vision 2, for about £40 all up including batteries and a charger!
Oh, and just a word when it comes to Lumens. Those P7 torches may boast 900 Lumens, but in reality the beam patterns are often too much of a flood to make them totally usable anyway. From what I’ve heard, most are more like 500 Lumens anyway (which is still a lot of light for the price indeed!). The smaller Cree units tend to be more of a spot beam (better for helmet mounted use) and throw a REALLY long way (my R2 will light up a small spot in the centre 50 metres away easily), and their quoted Lumen output is generally closer to that actually acheived.