I can say from experience in buying one that the mini was indeed hard to find and out of stock in many places (of course, depending on the version one wanted...there's different GBs, different colours and different wifi/3G).
I do know that when I got mine some weeks ago, it said over two weeks before delivery (perhaps because of strong demand versus production but who knows). So instead I tried to find it at a store by using the online locator at the Apple site. I was only looking for the one store I could go to, but it showed the 20 closest and they all showed as sold out.
I thought I'd call just to make sure and lucky that I did. I called at almost closing time and the store said they'd got a shipment in that day that wasn't showing up online yet and they still had a few left, I think it was two. They told me the online site would update at a certain time, and that if I wanted to secure it, I could wait til the site updated the inventory, and if they weren't sold out before then, then order it online to pick up at the store.
So that's what I tried. At the appointed time, sure enough the site updated and they were in stock, so I ordered and paid. Then out of curiosity I tried again to see if any were still available, and the store already said sold out again, heh. So it looks like I got about the last one. And not only that store, but again it showed me all 20 of the closest stores (heh, well "closest" is relative, they were pretty far away) and all 20 again were sold out.
The next day I went in and picked mine up by reservation, and all was well. Also, I had tried the Best Buy and they were sold out as well. Keep in mind I was only looking for the one I wanted - white, 16GB, wifi only - but from my experience a few weeks ago, they were definitely popular and selling well, at that time anyway.