Although I think they make one hell of a hot couple and I really wish they were together for real, I'm one of those who think they're just very good friends. Realistically, it doesn't mean a thing for best female friends to flirt or play around with each other; it just means that you're comfortable enough to have that kind of relationship.
For example, my best friend and I call each other "hubby" and "wifey", we flirt with each other, get jealous of other people, and are listed on facebook.com as "in a relationship". But we're not a couple and most definitely do not have any romantic inclinations towards each other. And yes, she does this with me even though she knows I'm like a 4 or 5 on the Kinsey scale. We don't bathe together, but that's a cultural thing... I'm sure that if we had been born and raised in Japan, bathing together would be completely natural. (Though I did shower with my female cousin till we were like 13 or 14. o_o)
Even for girls who are not close to each other, checking each other out, flirtation, and even making out is not uncommon. (How many girls do you know or have you seen that will make out with each other for money, to turn on guys, or for no good reason at all?) I think most girls would agree with me.
However, even after all this realistic rationalization, I still can't shake the feeling that Miki might be one of the few true lesbians in H!P. Bottom line, we'll never know, but it doesn't mean it's not hella fun to dream!