A comment someone made on the AVEN version of this thread reminded me of something that probably is significant regarding the bi vs. pansexual distinction. In the book
Sexual Fluidity, the author reports on a 10 year longitudinal study of women who had experienced some amount of same-sex attraction. The book mostly focuses on women attracted to men and women (largely because most research up to that point focused on lesbians despite bisexual women consistently being more numerous in survey data; however, the author avoids the term bisexual because of a lot of these women didn't feel comfortable using that term.)
Among these women, there is a group that feels they are "attracted to the person not the gender" and that gender isn't really a major component in their attractions (except possibly as a hidden variable of sorts, where they are attracted principally to people with certain sorts of traits regardless of the person's gender, though these traits may be more common in one gender than another.) Evidently, this phenomena pops up here and there in the literature, but is generally ignored or treated as an anomaly.
However, none of the women in the study identified as pansexual. For those who experienced both same and other sex attractions, a very large portion changed their identify labels at least once (often more) over the course of the ten years. Many were uncomfortable with "bisexual" because they felt it suggested interest in sexual relationships with men and women at the same time, when they generally (but not always) wanted a long-term monogamous relationship, recognizing that could be with a man or a woman. (Also, for these women, there often was a sense that "queerness" was, to a large extent a choice as they had the option of going with an other-sex or same-sex relationship.) Often, the identity trend was lesbian if they had relationships mostly with women. But then if they had one with a man, they might go to bisexual or unlabeled. One went from unlabeled to bisexual after getting married to a man, but that was because when she was younger, she had a number of lesbian relationships, and she was mostly attracted to women. But she ended up falling in love with a man and getting married to him. Because people tended to use this to dismiss her earlier lesbian relationships as "just a phase" (when she insisted her feelings were quite real), she identified as bisexual to make it clear that that had not been "just a phase."
Edit: I'm not sure what connection, if any, exists between the idea of "pansexual" and what Diamond (author of above cited book) calls "person based attractions" (which is a problematic term since people for whom gender is an important factor in their attractions also experience "person based attraction" within (a) gender/s.) Also, not all of the "bisexual" women experienced these sorts of attractions. Rather, some felt that there were certain mail traits they found especially attractive and other (overlapping but non-identical) female traits they found especially attractive. However, a big drive for "pansexual" seems to be that many people's politics motivates them to try to see people (themselves?) as post-gender, or rejecting a gender binary, etc. I really don't like the idea of attempting to enlist certain people's non-gender-based attractions in service of someone else's gender politics, as though these people have somehow "moved past gender" rather than that just being how they are. The only ideology I would be interested in enlisting these sorts of people's attractions into is one that says models of sexuality that can't account for them are too narrow.\
It certainly does complicated the idea of "sexual orientation", because it's not clear that everyone has one--including a number of sexual people.