I hope you are correct, that the upgrades COX did to support their Internet service has prepared them for HDTV. I know they installed a lot of fiber a couple of years ago, and new cable where it was required. It's probably a good bet that if you have Internet service, you may well be good to go for HD.
Prices on the cable boxes should come down pretty quickly. Remember cable modems were expensive at first. It wouldn't surprise me to see the boxes rent for $10 a month like the first cable modems, with an offer to purchase the box a year or so later. It seems reasonable to me that COX will invest in a few thousand boxes to (1) guarantee a supply, and (2) provide an option for people who don't have $499, or whatever the price of the boxes are. As availability improves and prices drop, people will start buying the boxes at CompUSA and returning the rentals, at which point COX will start offering them for sale.
Don't be TOO hard on COX for not offering the service yet. Mostly it has been larger markets that have rolled out the service, standards have only recently been set, prices for cable boxes have been high and supplies short, the first generation boxes are now being replaced by 2nd gen boxes which will have a longer "shelf life", and therefore be a better investment, and so on.
Up until now, DirecTV has offered only three HD channels, with two of those being extra pay channels. Now that they are offering ESPN, Discovery HD, and HDNet Movies, they start offering some competition to the cable companies. That's one factor. Consumer awareness of HD is improving to the point that most people know what it is, and are beginning to damand it. Prices on TV's have come down to the point where a critical mass of consumers can afford them. I don't KNOW anything, but I'm pretty comfortable in suggesting we will see COX roll out HDTV this fall. As has been suggested, September seems reasonable.
Now, if we could just get COX to share their plans with us instead of keeping it all a big secret...