Use your favorite unzip program (7zip, winzip, etc) to open up minecraft.jar. On windows, this is in the AppData. Specifically for me it is (windows 7):
In here you will find the dreaded terrain.png file, which is the heart and soul of the texture pack. A little more snooping will show you folders containing items, gui, mobs, armor, everything you need, depending on how far you want to go. As far as how you exactly structure the .zip of your finished pack idk, because I haven't messed around with it since before that was an option. But at least this should let you get the files and get to work.
C:\Users\*yourusername*\AppData\Roaming\.minecraft\bin\minecraft.jar\
As far as other os's I don't know.
In here you will find the dreaded terrain.png file, which is the heart and soul of the texture pack. A little more snooping will show you folders containing items, gui, mobs, armor, everything you need, depending on how far you want to go. As far as how you exactly structure the .zip of your finished pack idk, because I haven't messed around with it since before that was an option. But at least this should let you get the files and get to work.
~GLHF
http://www.minecraftforum.net/viewtopic.php?f=1021&t=136049