Your not barred from using the GPL version, but I think there are widgets or something that are different between the 2 and if you have developed the software using the commercial licemce you can\'t just compile it with the GPL version and release it as GPL - at least that was the arguement that the author of Qt!TreeExplorer put forth for not releasing the source to his free version. Seem that a lot of the Jaapnese developers have the commercial version of qtopia - maybe they had a sale on or something .
Doesn\'t make sense to me but then not many licence issues do
Stu