Chapter 9 of his Pninei Halacha: the Laws of Pesach has recently been posted to Scribd and it offers good news for chocolate lovers:
Chocolate and candy labeled “Kosher for Pesach only for those who eat kitniyot" are technically permissible even for those who do not eat kitniyot, because the kitniyot in these products are added before Pesach and are batel be-rov. In addition, these products generally contain kitniyot oils, which, according to several leading poskim, are not included in the custom to prohibit kitniyot.
He goes on to write that kosher certification agencies label them as "Kosher for Pesach for kitniyot eaters" because "people are stringent".
I disagree and believe that this is really due to the Charedization of the kashrut organizations--and it seems that the Tzohar Rabbinical organization agrees with me. Last year, Tzohar started a campaign for more accurate "Kosher for Pesach" labeling. (See "Vered HaGalil and the Kitniyot Problem".)
By the way, on page 7, Rav Melamed cites the source of the "kitniyot cooties" in chocolate:
The Badatz is strict about lecithin derived from rapeseed. Halakhically there is nothing wrong with this substance; however, there are many uncertainties that mitigate toward leniency. Firstly, rapeseed is not a legume (the technical meaning of kitniyot), but a member of the Brassicaceae family of crucifers, whose fruit grips thestalk and whose seeds grow in pods, much like the mustard plant. Oil is extracted from these seeds. According to Igrot Moshe OC 3:63, we do not forbid anything that was not explicitly prohibited by custom. Additionally, it is debatable whether the status of kitniyot can be applied to the seeds of a plant when it is clear that the plant itself is not kitniyot....For more information go to Pninei Halacha: the Laws of Pesach (Kitniyot).