A lot is written about ionic hair dryers but the real scientific background indeed is slim.

Like someone else suggested in a forum: http://ci.nii.ac.jp/naid/110001080304/en/
and I quote:

A small article is found here about negative ions in general http://www.negativeiongenerators.com/proofpositive.html

Seems to be even more popular in Japan than in the US. Another article on thefreelibrary.com seems to show that these ion generating claims are just sales tools...

Since this topic started, there still is no real proof that this is working. It does work against static hair and it seems to work as a placebo (but is it?).