LTE is coming to at least the VZW version of the next iPad. This much I know and given that it's likely a single platform for both carriers, it stands to reason it's coming to AT&T at the same time.
On this you are either right or wrong. We shall see.
This perceived lack of LTE chips and network isn't keeping HTC and Samsung from selling the heck out of their 4G devices.
Here you are entirely off. 1. There is no perceived lack...the lack is genuine. 2. What data are you seeing that shows HTC & Samsung are selling a lot of LTE devices? 3. The biggest difference between Apple and those manufacturers, is that Apple doesn't pack a premature feature into a device just to sell it and drive numbers out. The few people who DO fall for the current LTE marketing campaigns find out quickly, after purchase, that LTE coverage is sparse and not worth paying for, and it saps your battery life like no other. That is the experience currently being sold by device manufacturers forcing LTE onto the market before its truly ready.
I would say that using that as an example of why Apple will include LTE sooner rather than later, is about as wrong as one could be.