Single PNG Button Rollover Question

[MadBnz]

Simple question, When creating a Rollover button, Do you have to export the button in its two states and change to Gif/Jpg? I would prefer to be able to save the button in native PNG format and then have the page call the correct frame of that PNG

So basically instead of having:
Button.gif <-Normal State
Button_f2.gif <-Rollover State

Can you have:
Button.png,1 <-Normal State
Button.png,2 <-Rollover State

I dont want to use pregenerated Fireworks Html, Just want to know if I can reduce my image Preloads by halving the number of files.