The iTunes store has pretty much everything in widescreen that it can, but as with most televisions and "home theater" setups... Widescreen really doesn't matter. I know that some people prefer widescreen, but I don't. I have a projector not a TV so I have a 20 foot screen and the black lines in most "widescreen" movies eats up 4-5 feet of space. The only movies that come formatted for a widescreen TV in the first place(I am talking about on DVDs) are the Anamorphic Widescreen DVDs. They have a 2.35:1 aspect ratio, but very few DVDs actually have this, most are in 1.85:1 aspect ratio or 16:9 which is actually a fullscreen movie with black lines at the top and bottom.
As for HD rentals in the iTunes store, this is AppleTV only, and that is not Apple's fault, this is due to DRM. In order for you to play a copy-protected Blu-ray or HD DVD you have to have
http://en.wikipedia.org/wiki/HDCP compliant hardware, the apple TV is HDCP, but you computer is not, so therefore Warner Bros/FOX/Sony etc. Can not be guaranteed that you won't be able to pirate their content.
Most people do not have a widescreen TV, and yes most Computer screen LCDs are capable of just about any resolution, but a lot of people are still in the non-HD zone.
I agree that many people want HD content, however given how hollywood lost the battle to keep DVDs copy-protected, they have taken enormous steps to protect HD content.....