I'm going through another playthrough of Pokemon Diamond (getting ready for X and Y of course), and had one open spot in my party by the time I got to the Trophy Garden. My first encounter was a Pikachu (10% chance) and seeing as I didn't have an Electric-type yet, since I can't stand waiting so long for Shinx to learn Spark, I went ahead and caught it. To my surprise, it was holding a Light Ball (5% chance). The only way that could've been better was if it was shiny, but we can't have it all.
So I encountered a Pikachu that only appears 1 in 200 times (1 in 400 if you count the gender split, mine was male), and come to find out... Poke Transfer doesn't allow held items. So, I'm just gonna have a beast Pikachu in Diamond, because he can't bring his Light Ball to B/W/B2/W2 or X/Y. Maybe I'll luck out and get one in Y.