And the analogy is so neat - it would make Doc Getzin proud. Not that you know who that is ;)
Bisexuality assumes that gender is digital: black or white, 1 or 0, male or female.
Pansexuality encompasses the fact that gender is analog: that it is a wide, wide range with male at one end and female at the other - and an infinite number of points in between. Certainly a better word, at least for me :)
Maybe this isn't the newest of thoughts, but it's new to me, and I thought it was so neat I wanted to share it :P
And I don't mean to offend those who identify as bisexual - I'm just trying to state how, for me, it doesn't work as a label.