I think birth does weird things to people. I would definitely be very dissapointed. I actually had a friend, who is very close, but wasn't one I was planning on asking to be at the birth, offer not to take off for T day in case I wanted her, so I do think that it's completely reasonable for you to expect her to change her plans.
That said I went through a lot of weirdness with another extremely close friend around difficulty TTC, and having m/c. It turned out that she had issues with not wanting to be a mom, and couldn't really relate to my desire. I didn't really expect that to matter, but it fed into everything in our relationship until well into this pregnancy.
So this may have everything to do with her own ambivalence around birth. She may want to be pregnant, or really not want to be pregnant, or have issues with her own Mom that are effecting her behaviour toward you.
It sucks but I think you've got to nail her down, and tell her how important this is to you,(and why) and how you can't really hang with maybe she'll be there and maybe she won't. As scary as it is to do this, it's really better to know one way or the other whether she's willing to be there for you, then to let her off the hook, by allowing her to misunderstand her commitment. I've often done that and I always regret it.