I don't know, but I saw at Snowpoint City today the Diamond Dust again. My DS time was set to Wednesday April 22, 2009, in case it matters. (I set it a day back, don't know why). Um, is it supposed to get Diamond Dust again?
If you look closely below the gym, or near the tree to the right of the bottom, you can see Diamond Dust. Also, on the Television Screens it was showing the weather program, and there was information about the Diamond Dust.
3DS FC: Snail 4527-8592-7979