hey there,
for all intents and purposes they are essentially the same chip, mainly, as has already been pointed out, the clock circuitry was revised, along with a few other little tweaks internally. Code writen for one, will run ok on the other, unless you are specifically wanting to use the extra clock speed for something. Below 10Mhz, applications should run equally well with either chip, assuming that you are using the 10Mhz version and not the 4Mhz version of course.
I seem to remember there being a slight difference also in programming spec, but it's probably a timing thing, most of the homebrew style programmers won't have an issue with it though, I don't think, because their tolerances are pretty wide. My Superpro programmer for instance, demands the correct chip be selected before it will begin to program it, but it has a smidge tighter tolerance than the average simple pic/eeprom only style programmers.
hope this helps...