I just ordered this one:
Meike FC100 LED Macro Ring Flash Light for Nikon D300 D3100 D90 D80 D7000 D5100 | eBay
It says "for NIKON", but I have answers from a chinese seller as well as a polish trader which both state that distinguishing between brands is just a marketing thing. No matter whether they are labeled as "for Nikon, Canon, Sony, Pentax,...", they all just use the x-contact.
If you have a closer look at the specifications, this makes sense. The LED flash offers variable flash INTENSITY, the ordinary flashes offer mainly variable flash TIME. The flash time of the LED flash is fixed at 1/100 sec, which seems to be the limit of the used controller. The LEDs itselves should be able to handle shorter times.
Using pTTL (not TTL!) could enable a LED flash to control flash intensity, and maybe flash time as well. But then the flashes would need the same electronics as the "real" pTTL flashes (minus the thyristor and the high voltage capacitor). But then I think no one could offer it for GBP 21 including shipment from China to Germany.
Besides the fact that you cannot use it for fast moving objects, the only minor problem I see is that the bandwidth of colour temperature maybe much bigger than with flash tubes. Of course never as big as with natural daylight, which is a mixture of all frequencies. With bad luck, the 2 sides of the flash could offer significant different colour temperatures, which could make WB somehow difficult. If it is roughly the same on both sides, it could be checked once with a grey card, and programmed (with my K200D, I could overwrite one of the seldom used WB presets).
Last edited by RKKS08; 03-22-2012 at 12:36 PM.