This question actually was brought up by Stephan Jung on OpenEEG mailing list, and I found it quite interesting.
Neurofeedback is built around reading person's brainwaves and presenting them back in perceivable form - so that, for example, game character moves faster for one type of brainwave patterns and slower for others, so person can perceive from character's speed what type of brainwaves currently flowing from his brain. And we know from neurofeedback studies that if done for period of time long enough, this can lead to permanent change of brainwave patterns, which in turn leads to change in behavior, mood, attention and etc.
But this - reading brainwaves & presenting them back - is what actually some types of BCI do. So prolonged usage of such BCI while playing could lead to unpleasant changes in player's brain functioning.
EEG based BCIs pick up waveform patterns from the EEG to work as command signals. Originally, in nature, these patterns result from different brain states (which can also translate to "mind" states) and with sufficiently prolonged use of these states for biofeedback, the brain can be made to "learn" to bring these states on at will. I am not sure if this can also result in permanent (or at least sufficiently persistant) changes in EEG patterns but this definitely sounds plausable. What may result from these permanent/persistant changes can, in theory, affect the person's thought process or mood.
Any new technology is only as good as the use we put it to. Effort and research towards the use of these EEG based BMIs to drive a robotic arm to function in place of an amputated limb or to drive a wheelchair for the paralyzed individual is understandable because the benefits far out-weigh the risks but are such devices really being used in any commercially directed games etc? I just read your update on NeuroSky and StarLab hardware. If the motivation behind these inventions is commercial, then the developers need to re-consider the use of this highly complex (and maybe dangerous) technology for things like gaming, that can not only be addictive, but targets the youth!
This reminds me of all the stories about projects like MK-Ultra etc. And then there was a similar furor a couple of decades ago about the subliminal stimulation technique. I wonder what happened to all those lines of research.