Hi,
I am a grad student studying in Taiwan. I just downloaded Pleco for my ipod and am really impressed with it, now just working out which components to buy. I saw someone using Pleco on the Palm last year who raved about it, but I couldn't really justify buying another device at the time. So I am psyched to have it now. Until now just been making do with the other apps available for Apple devices.
I noticed a lot of people have posted ideas about a Classical Chinese dictionary for Pleco. The 王力 《古漢語字典》Wang Li Guhanyu Zidian has come up a few times in various threads, and I can't recommend this dictionary enough for anyone who studies even a little Classical. This dictionary gives accurate, concise Modern Chinese definitions for the different senses of a word, information about etymology, and it often tells which senses of a character developed later in history. It does all this without being overwhelming. Basically, Wang Li is an amazing linguist, he compiled a whole series of books for teaching classical and another famous dictionary, the Tongyuan Zidian. Anyway, I have the Hanyu Da Cidian (and the Zidian) on my computer, but I actually find myself reaching for the paper copy of Wang Li's book more often. I would pay lots of money (at least by grad-student standards) to have something like this on Pleco, or some good quality dictionary for learners of Classical Chinese.