I've been following "RadioPopper" development for a few months now and I reckon they look pretty spiffy! They have just release more details and photos of their "P1" (One channel ETTL/CSL) units.
The basic idea is that they take the optical IR signal from an ETTL/CLS master unit and transmits it to the slave flash using a radio signal, thus greatly extending the range and improving line of site issues typical with IR communication.
Unfortunately I don't have much use for the "P1" units as I don't have an ETTL master (and only the one slave) but I am definitely looking forward to the release of the RadioPopper Jnr units. They will be a simple radio trigger using a similar design (without the optical stuff) that will hopefully be more reliable and have greater range than the GI triggers for a similar price.
To stay up to date on the detail have a look at the RadioPopper Blog.