In the NTSC Television format (60i) 1 frame/image consists of 2 fields that describe the picture. 120hz is just a repetition of the same fields. For static images this looks like an improvement. However, fast moving images could give some unwanted side effects because of the repeating double fields.
So for me no need for 120hz, it's just marketing..... For the time being I will stick to an old fashioned TV instead of LCD/Plasma, untill full HD screens are affordable.