I'm with you, though I'm fortunate to be in a group with a lot of other SAHM's, so we get to commisserate on that, and I don't get too much of the dropping kids off or cancelling (other than kid issues - upset tummies, temper tantrums, that sort of thing). It's the other people, who aren't SAHMs right now.
My mother-in-law (who was also a SAHM when her kids were young!) sometimes calls me to do things like make sure her garage door is closed or double-check her crock pot (she couldn't remember if she'd turned it on or not). She has asked me "I decided to change my flight - and I don't want to take a cab - can you take me to the airport? In an hour?" She did this VIA E-MAIL. She only communicates via e-mail. (I only saw it by accident - a girlfriend was helping me set up some new passwords on a different site.) Another girlfiriend calls me several times a day - when she's bored. Because I'm home, right? Now, I don't mind talking on the phone while I'm doing other things, but it's not always easy, and now that the kids are older, they need more time and attention. I've started ignoring about half of her calls.
I've also told my mother-in-law that I will run the errands at her house only if the kids and I are going out, anyway. (It's not usually a safety issue - you can't get into the house, her garage is detached.) I've also pointed out to her (several times) that I'm doing other things besides sitting in front of the computer all day, and if she wants to reliably reach me, she'll have to call.
Peopld break plans with us sometimes, too. We usually just go anyway, with or without them.
My advice? If people show up unexpectedly, answer the door while wearing shoes and holding your car keys - tell them you're on your way out. (And then go out! Library, pool, park, McDonald's, whatever.) If people drop off their kids, start telling them they need to drop off snacks, and drinks, too. (Lie, if you have to, and say you haven't been grocery shopping.) I imaging babysitting everyone's kids gets expensive.
Hang in there, lady. We know we're important, right?