Well, you had a bit of a long wait but the supply isn't Acer's fault, it's the fault of the manufacturer of the PDA which has failed to keep up with demand - annoying but not uncommon at Christmas.
I would have thought Acer calling you and telling you they weren't going to get the PDA until the middle of January is perfectly acceptable - you just jumped the gun because your revised delivery date was December 12. I doubt it was "just an excuse" - businesses hate losing customers and the aim would have been to encourage you to keep your order open by keeping you up to speed on the status of your order: something to be commended, I think. And they were going to let you know a full 12 days before Christmas - you probably could have ordered from somewhere else pretty much in time for Christmas if it was a gift.
You also can't expect the sales team to be making minute-by-minute changes to the website - if he said he'd get it changed, I'm sure it will have been changed. It's not in Acer's interest to annoy customers.
I think you've had an annoying and frustrating experience but I also think Acer has behaved as well as can be expected.