Hi there, and I have been goofing around with Minecraft, and I came across this epic glitch, on getting infinite diamonds -- For 1.2.5! So use it before the update =D
NOTE: This only works in singleplayer so don't tell me it doesn't work in multiplayer!!!!!
So first you want to acquire at least 1 diamond.
Next you want to throw all you diamonds (Or diamond)on the ground
Then, you save, and quit to title. Do not exit out of Minecraft client, only exit to the title screen.
After that, log back into the world that you threw the diamonds on the ground on. Then pick them up, and once you do, open up your inventory, and close out of Minecraft client completely, by clicking the red 'X' button on your Minecraft screen. (If you do not see this button, hit the F11 Key on your keyboard)
Now, re-open Minecraft client, and log back into the same world, and walla! You should have double the diamonds.

If I have helped you out, don't forget to leave a diamond, or a rating.
Lets see if we can aim for 5 diamonds! =)