for cfls the most useful kelvin temps are 6500k and 2700k. 6500 is the best cfl for representing the blue spectrum like a MH or mercury vapor while the 2700k is closer to an hps though i think hps are 2100k while i think MH are 6700k i have 350watts of 6500k cfls in my veg room and one stray 2700k cfl
honestly i dont know the specs on it. but i know that before cfls were the new thing, all that was around were regular long tube fluoros. and when using those 'cool white' was always a no no.
so i guess it just kind of stuck for cfls too. essentially im sure a cool white cfl would still work. but i know i would personally feel alot more confident getting ones that say sunlight- or daylight, etc. you know?