Seamless loading is one of those \'holy grail\' type things. The major issue with this is the fact that everything has to be thread safe ( i.e, you need to load stuff in the background and for that you need a loading thread ). Making things thread safe is *very* difficult. Anything to do with threads is difficult because of the processing is not really consistant. Depends on how the threads share the CPU.
Believe me, I\'d love to have seemless loading. WoW is awesome in that way, even when you are on the taxi you don\'t notice any loading and hardly any popup.