Maybe you know this wonderful video:
Can someone help me to understand how the LED trail effect has been done?
There are two ways this can be accomplished. The easiest is by using a slow shutter speed. While most DSLRs may be limited to 30FPS and faster, it is possible through the use of third party firmwares to lower that arbitrarily further. As you can see from the details on Vimeo, they were using GH2s with the EOSHD Vanilla hack. This removes many of the limitations that would otherwise be on the system. It is also possible with most Canon hardware using MagicLantern's FPS Override.
The other way to do it is to artificially do it in post production (though the result often isn't as good) by additive combining each frame with previous frames. This results in a trail that can animate on and animate off and maintain the original frame rate, but it is likely to introduce gaps in the trails and either requires a pre-existing plugin or some creative layering in After Effects. Layer each video one from apart and then apply blending until you get something that looks like you want.
It is hard to tell which technique is being used for sure without a downloadable copy that I can go through frame by frame, though my initial impression (particularly based on the hacked firmware and the general feel of the video) is that they are using the lower frame rate option in the hacked GH2.
Update: I managed to dig up my old Youtube downloader and got the video in to Premiere. I can confirm that it is in fact a lowered frame rate (around 1/12 or 1/15 for most of the trail shots).