Way too much work converting Pleco from C/C++ to Java, which Mike has mentioned in other threads. It also creates all sorts of wonderful maintainability problems: every bug would have to be found and fixed twice - once in the C code, and once in the Java code, along with having to code every new feature twice (again, once for C and once for Java). Add to that the probable lack of a touch screen, and the amount of work to convert Pleco to run on Symbian is pretty insane - possibly a couple of years of development time...
EDIT: Assuming that phone runs Symbian. If it doesn't, then I'm not even sure if the processor speed and memory are sufficient to support Pleco...
EDIT 2: Not trying to say bad things about your choice of phone or anything, just pointing out that Pleco is quite resource-intensive for a Mobile application.