IMHO, I don't believe it will take very long for the other major networks to jump on board. As we all know, the bulk of revenue for the networks comes from advertising. If they jump into the online rental space, they won't be able to charge as much for advertising. That means that the revenue from online rentals would have to, at the very least, offset this lost revenue from advertising. This can only be done if there's a large demand in the online rental market. That remains to be seen, but Apple is off to a good start with getting ATV's into homes at the rate of 1 million per quarter.
The last several years, since ATV v.1's release, has given Apple a very clear picture as to the price points that will entice consumers. I think they've learned that the $2.99 price to purchase a newly-released HD television episode has not caused consumers to open up their wallets. However, Apple now believes (as I do too) that the $0.99 rental price will capture those "impulse purchases". They're hoping that the iTunes rentals become the equivalent of the gum/magazines at the checkout aisle.
Personally, I've never purchased a television show or movie off of iTunes. However, I have rented two shows in the last two days (both "Top Gear" episodes). As a disclaimer, I'm one of those people with digital cable and all the premium channels. So why did I rent these episodes if I have so many other options to watch? The reason is because my cable provider doesn't offer BBC America-HD (they only offer BBC America in SD) and I wanted to watch the show in HD. Do I miss the $2 I spent on those shows? Not at all. Do I wish these shows were purchases so I can watch them again? Absolutely not. To the contrary, I'm actually waiting for them to expire and fall out of my queue.
I think we'll eventually see a domino effect with the networks coming on board. Do I expect CBS and NBC to be the first of the dominos to fall? Not a chance. What I think we'll see is, not the major networks, but rather other premium content providers entering the rental market. I noticed that the Food Network was recently added to iTunes for episode purchases (not yet rentals). The Food Network is owned by Scripps, who also owns HGTV, the DIY Network, the Cooking Channel, the Travel Channel, etc. If I was Apple, I would focus on getting these kinds of companies on board. If they add rental content from Scripps, another network like Discovery (who owns the Discovery Channel, TLC, Animal Planet, etc.), and AETN (who owns A&E, History, Lifetime, etc.), they would fill out their iTunes rental store quite nicely. This additional content would only draw more and more consumers to the rental model, so many in fact, that the major networks will not be able to ignore them. A funny thing about the major networks is that they already give their shows away for free on their websites or OTA. The only difference is these "free" shows come with advertising.
Of course, there's always the possibility that I'm overly optimistic and my ATV ultimately ends up being used as a paper weight. That being said, I'll gladly place my $99 bet down on Steve Jobs and Apple.