Novak's site mentions a bit about esc frequencies:
http://www.teamnovak.com/Tech_info/esc_termin/
http://www.rccentral.com/guides.asp?...11&PAGE_NUM=38
For the same esc which can switch frequency, I feel that the lower freq e.g. 2k gives stock motors more punch, and the high one I got 4khz gives a very very smooth, linear throttle reaponse.