Gursharan Singh's answer covers all the bases. This Bola - Greeting is for ALL and conveys that ALL is Waheguru's Posession. The Khalsa is of waheguru and the Victory is also OF Waheguru.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sidh-gosht/5308-sidh-gosht-pauri-1-guru-granth.html
GO ahead and reply to this Gurfateh with VIGOUR. We need to make this our Normal greeting. "Sat sri Akal" is ok only to the extent if you understand that it is only HALF a greeting...IT is actually a KHALSA WAR CRY JAIKARA....and the Complete verse is JO Boleh SO NIHAAL..SAT SRI AKAAALLLLLLLLLLLLLLLLL. You will notice that the War Cry goes UP in Volume and Pitch...and that is why it strikes TERROR in the hearts of the enemy. Whenever the enmey hears this war cry...they rather skirt around and take a different route !!!
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=5308
jarnail Singh.