... I am SO irritated right now...
I didn't know for sure if I was doing the Dove promotion as of Thursday morning. The college professor called up before work, and wanted a definite "yes" or "no" for Saturday's art class. I said "yes". Paying work.
While I was at work, the promotions company left a message confirming.
As soon as I got off work, I tried calling, couldn't reach the promotions dude, and sent them an email saying I couldn't work for 4 hours of it because I was doing some other work.
I didn't hear from that company all last night.
I arranged another paying gig for Sunday and a tfp with a photographer whose work I admire for the rest of Saturday afternoon (and who then made plans to drive here from Toronto).
This morning, I had an email in my box from the promotions company saying that I should have called them sooner and asking if I could work Sunday and Saturday afternoon...
... yesterday, before 9pm, I could have said "yes" to that....
I still haven't answered their email and I'm not sure how to. I feel like a flake, because I did say "yes" that I really wanted to do the work... but I can't turn down other paying work and flake on other people just because some promotions company run out of someone's home can't get it's poop in a group and is inefficient!
rrr... sorry for my rant... anyone have any suggestions how/if I could have handled this differently? Has this happened to anyone else?

Sounds like you never gave a confirmation to work.  You just expressed a desire to work.   That is fine.  But your disorganization seems to be only hurting you.

You need to contact him and let him know how many days in advance you are usually booked. For me as a photographer I warn people that weekends are almost never available but if you want one call me either a month in advance, or the week of to see if I have a cancellation,

Star

You see to be between a rock and a hardplace. I'll assume both gigs are paying, if that's the case, you have to determine which pays more, or which will get you the most *valuable* exposure.

Interested to know what ever happened with this situation?

For certain projects I utilize a booking agent.  The steadfast rule is this, it goes in the book in pencil and is contingent on first in with either a confimation e-mail, fax or deposit...then the date gets overwritten in ink.

It's never a problem so long as those inquiring for service understand the booking policy.  As for missing out on higher paying gigs?  Well, it's the nature of the beast once the committment is made, a small price to pay for not gaining a rep going against my word.

I use to book lots of promotional work for models.
The companies always contact you at the last minute, and want a million models, that they want to meet, and pick from.

Half the time, the contact person is never around after business hours (9-5) and can not be contacted till the next day.  It is quite an undertaking trying to get a lot of models together at the last minute due to them usually working other jobs.  It takes about 2 weeks to do properly, so I request 3 weeks before the event.  In the end it doesn't matter what you ask for, they always want someone last minute, and get mad when you tell them you need to check schedualing.
Then when you try to reach them, the client is never around.

I always first give them my cell phone number that is always on, and they can call me any time 24 hours a day.  Instead, they  call the office (yellowpage) phone repeatedly when i never gave them that number.  I usually am at an assignment in some other city than the office so I am not there answering that phone, that is why I gave them my direct line, that they don't call.

Anyone who knows me, knows that I am very easy to find.

It got so bad, that I gave up on the whole demo thing.
Most of the models I work with don't consider it "real" modeling anyway.

I use to feel bad as you are feeling, but as long as you made your best effort at contacting them, and letting them know your intentions, I would not feel bad.  From what you said, nothing was solid.

Clients need to set things up 3 weeks in advance, or for me, it's a crap shoot....and I let them know it.