I don't think apple holds back the supply of products just to increase demand. Besides I wouldn't even call the iPod a hot product, the reason being is because most people already have one (I am obviously the exception, but I will have one when I get my 4th Gen touch). Its just simply a new generation of a product that already exists. A new product like a toy or a video game system that did not previously exist, now that is a hot product.
Video game companies do make limited supplies of consoles in order to boost demand especially at launch, I see lots of stories about lines forming in stores for video game consoles, but the new iPods have not gotten any news coverage yet (despite several stories on the halo launch yesterday).
If iPods are out, I would expect them to come back in stock in a week or 2 or perhaps sooner, they aren't going to be out for months like with certain game consoles.
Retailers on the other hand, they might hold back the iPods for launch on a certain day in order to drive more traffic to their stores, but that is purely the doing of the retailer, not apple. This is a dirty tactic to increase sales and I try not to support retailers that are clearly guilty of holding a product until a certain day.