Fares from LA and SF to Las Vegas definitely show the effects of Spirit's presence. I wish Spirit would fly SF area to LAX, but I think they realize that Southwest would defend that market to the death.
Southwest will occasionally match NK and F9 on HOU-MDW/PHX although there doesn't seem to be a lot of rhyme or reason to when they do. One day I'll look and the WN fare will be around $37 then then next day it's back up then two days later it's down to $39.