There are a number of factors that would determine my level of interest in a Pleco desktop. At present I use Wenlin which has several definite advantages over Pleco. First, being a desktop app it is very good for typing Chinese. I use it for composing all of my assignments in my Chinese class, for composing emails, basically anything I write in Chinese I do using Wenlin. Second, it provides an historical analysis of characters that I find very helpful in learning new characters and understanding their etymology. It also has other features that make it useful. There are two big downsides with Wenlin however. It has the ABC dictionary and a small, not very useful English-Chinese dictionary and that's it. So Pleco's dictionary support is far better. Second, there is no mobile version of Wenlin and my understanding is that there probably will not be. Indeed, Tom Bishop, Wenlin's developer, referred me to Pleco when I asked him about a mobile version of Wenlin. Third, Wenlin's flashcard system is far inferior to what we have on Pleco.
I think Pleco has the handheld market sewed up. At this point my main interest in a Pleco desktop would be access to more dictionaries than are available on Wenlin, esp. English-Chinese. But my understanding is that Wenlin 4.0 is going to be released soon. I don't know what they have in mind for this new release, but if it is a very significant upgrade it might make a Pleco desktop version less compelling, ESPECIALLY if Pleco is released on the iPad, which seems very likely, and if Wenlin is not available on the iPad, which also seems likely, at least for the next release. I will almost certainly buy the Wenlin upgrade when it appears. Whether I would ALSO buy Pleco desktop would depend on whether it offered significant features that Wenlin 4.0 would still lack, and which I would want on my desktop as opposed to on on iPhone or iPad. I think that the iPad will make a very good platform for reading and for work with flashcards. If they significantly improved their dictionary support AND if Pleco were to become available on the new iPad, I would probably opt for Wenlin 4.0 for my typing and "research" needs (I don't think I'll want to do a lot of typing on a iPad) and Pleco on iPad for reading and flashcard practice and continue to use Pleco on iPod Touch or on Windows Mobile when I am away from home. At that point I don't know if a Pleco desktop would add anything vital. Basically I think the release of the iPad changes the picture significantly. Given that Pleco is already running in the iPhone and iPod Touch and the potential that the extra screen real estate has for significantly enhancing Pleco's usefulness as a study tool, I am at present more interested in an iPad version than a desktop version, esp. in light of the imminent upgrade to Wenlin 4.0. The large format touch screen of the iPad just seems to make so much sense as the next step for Pleco. I think an assessment of the potential market share for a Pleco desktop would have to take Wenlin 4.0 into account, whereas I wouldn't expect Wenlin 4.0 to compete as directly with Pleco iPad.