As for regular rentals, by Macworld's calculations, around 400 titles are available (my own calculations show 417), which isn't a substantial increase from my own calculations back in January. The total number of available films, for rent or purchase, available via iTunes is teetering around 800 by my calculations. Clearly, this is well short of 1000 titles.
So the question becomes, why is there a delay? Some of the editorials that have appeared online seem to take the position that Apple failed to meet predictions in part because of delays with the Apple TV software. While I could see how that might affect the number of available HD titles (and remember, HD titles are much closer to meeting expectations than regular rentals), this just seems too convenient.
It has also been inevitable that parallels be drawn between the number of available movie rentals and the delays with the iPhone SDK, as if there is some sort of pattern in Apple not meeting expectations. To me, this makes absolutely no sense. As far as I can tell (and granted, I'm no expert), the iPhone SDK is wholly unrelated to iTunes and the iTunes rentals. The fact that both systems have had logistical problems with roll out is unfortunate, but I can't see how they are either related or indicative of a trend.
So what's the real story? While I doubt we will ever get a definitive answer, I think Ars Technica has the most plausible theory: licensing issues with the movie studios. It's no secret that the studios have been reticent to fully embrace digital distribution. Personally, I think the fear that digital rental sales could cannibalize home video sales is understandable. Home video attributes for a huge amount of the studios return on investment and embracing one model at the risk of another must be scary.
Plus, it is clear that Apple had to cede to the studio demands just to get the rental agreements in the first place. True, all the major studios are represented, but certain production companies still have the right to refuse to appear on iTunes. Apple also conceded to flexible pricing and a 30-day exclusive DVD window (if the studio chooses to invoke it). Apple does not possess the power they have in music sales in the video space, as such, I find it more than plausible to believe that they are more at the studio's mercy for when films will be available and on what terms.
Undoubtedly a large library was one of the keys to iTunes' success in music. It is crucial that Apple achieve something similar if they want rentals to succeed. It's easy to blame Apple for dropping the ball, but in this case, I really think this is something that is out of their control.