Hmm, the cannons on the ships _did_ use gunpowder or sth. similar in WC. But it also is my feeling that hand-held guns, as opposed to cannons, do not _feel_ right, despite their existance being justifiable. However, an average handheld gun (that could be justifiable) would have roughly the same power as a crossbow but fire _way_ slower than a bow and also slower than a crossbow and be less accurate than either crossbow or bow. However, a quickloader _might_ be justified, i.e. one in which bullet, powder and ignition go in seperate ways but at the same time, using three magazines. This would be a high-tech top-notch gun, however, and be very very hard to come by. I wouldn\'t like shells, though. Furthermore, any gun would be rather unreliable considering environmental conditions and manufacturing tech.
I would second the \"no electrical devices\" bill (with special exceptions for Frankensteins\' or other room / building sized devices).
Summarising it is more the transportability that makes one not like these techie things in a fantasy setting, probably because we all are used to this being the most obvious and well-known difference between modern day and medieval times.
While magic _could_ (and probably did) speed up this process, we are setting PS in a medieval world, thereby implying that, even magic, the sciences have not yet developed that far. This is entirely plausible as the population is quite limited (fewer brains -> fewer inventors -> fewer progress) and the world is still relatively young.
Therefore, ther is no contradiction in the PS world being young but already having medieval-tech.