The sikha reportedly signifies one-pointed (ekanta) focus on a spiritual goal, and devotion to God. It is also said that the sikha allows God to easily pull one to paradise, although this belief is unsubstantiated and maybe a more islamic belief. According to Smriti Shastrasa it is mandatory for all Hindus to keep sikha and the first three twice-born or Dwija castes to wear Yajnopavita or Janeu or Paita (sacred thread). For more information see: http://en.wikipedia.org/wiki/Sikha
* Basically the different sikhas sported by Vaishnavas denotes their Sampradaya (just as Tilak does) and therefore what part of the Yajur-Veda they follow, whether it be the Shuklam Yajur-Veda the North Indians follow or the Krishna Yajur-Veda the South Indians follow. All styles of dress, appearance and mode of worship are dictated by geography, demographics and the Yajur-veda! ;-)
