John C has asked me to post my own beliefs of what makes something internal or external, so to the best of my ability, here goes...
First off, when I don't understand something, I try to not have a strong opinion about it. If I am not informed about a political issue, for example, I try not to have a strong opinion based on the words of some charismatic leader, or based party on some ideology. It can be hard to live with one's own ignorance, but I find that it is the easiest thing for me. Bearing this in mind, I don't have a strong opinion on what constitutes "internal" or "external."
At this point in time I tend to regard "internal" arts as those with a major focus on training that has similar methods and ends to some of the esoteric types of yoga. I believe that heavily internal schools tend to focus on generating, manipulating, sealing, and focusing esoteric energies (chi, ki, prana, etc), and directing it along with physical movements that are heavily practiced in coordination with yogic-like breathing, body control and coordination, often done very slowly.
Obviously, some Karate styles do this sort of thing. Sanchin (I believe) coordinates internal power through very controlled breathing, developing yogic-like muscle control and coordination, and focuses internal energies in particular parts of the body. I have heard that different Karate styles focus this generated energy in differnt places, based on the prefernces of the style.
More clearly "external" arts (like boxing) also focus heavily on body mechanics, balance, and to a degree breathing, but yoga-like methods such as training very slowly and with very directed tensions are generally absent from boxing, and breathing techniques tends to be much less sophisticated. The mystical aspects found in Muay Thai and some Filipino arts tend to not be yoga-like in nature, but tend to be based more on the manipulation of magical symbols or appeals to external supernaural forces. As with boxing, body mechanics, balance, and breathing tend are very important in these styles, even when they are not taught within an esoteric framework.
I'd say that the more an art trains like yoga, and the more it deals with esoteric energies, the more likely it is to be internal. I believe that a person can practice the external techniques of an art a predecessor once taught as an internal art, and if they don't understand the internal background of an art, and don't teach the yoga-like underlying methods, they may be pretty-much practicing an external art even if they think their art is internal.
This is largely speculation on my part though. The arts I study (as far as I know on my level), don't tend to look at themselves withing an internal/external context.