Well, I'm glad to relieve you of the worry that it was a glitch, because the first time I EVER saw it, I thought the same thing. I'd forgotten about this thread until now! Yesterday (I say yesterday since it's past midnight here now) on my Diamond, I was in Snowpoint and the diamond dust effect was happening again. So, I checked my Pearl game after that and...sure enough, it was happening on there too. The thunderstorm thing wasn't happening though, but with that and with whoever got a thunderstorm on their game along with their brother, it just gives me even more evidence that there's certain dates that it'll happen.
If that's correct, then that must mean diamond dust occurred on everybody else's games yesterday as well. Or if not that, maybe it targets people grouped together in possibly a 10 to 30 mile radius? It might be more complicated than it seems. Oh well. Even if it doesn't do anything it's inspired me to include it in a story I'm writing ^_^