The iPad doesn't run Flash. If your website uses Flash, it won't play well on the iPad. Turns out, a lot of people want their sites to look pretty on the iPad.
One of the more interesting effects of the iPhone was that it drove a ton of websites to format their content for the phone in at least of two ways, and often both: iPhone-optimised sites, with more finger-friendly navigational elements that look almost app-like, and actual iPhone apps. We're seeing a repeat with the iPad, though the adjustment appears to be less about the screen size than its lack of Flash support, and there's the fact a lot of sites will be ready on day one. (Though before we go any further, let's be clear: Flash is sticking around, for many reasons, regardless of Apple's opinion of it.)
The NYT, WSJ and NPR are all following the twin attack: They're launching iPad-tuned homepages that dispense with Flash entirely, with layouts designed to be held in your hand, like the front page of a newspaper, and they're coming out with iPad apps. The WSJ app will run $US17.99 a month - $US215 a year - a seemingly ridiculous sum, since subscribing to both the print and online editions is a mere $US140. We don't know what it looks like. Interestingly, while we've seen the most of NYT's iPad app out of anyone (presumably), we don't know how much it's going to cost. NPR's app is a free iPadded version of its iPhone app.
There's also the video services, which are, in a way, basic internet infrastructure. YouTube, of course, has been playing with HTML5 for a bit, as has Vimeo, and both have served up iPhone OS-tailored video for a while. Brightcove, another big video service, used a by lot of magazine sites (Wired, Slate), is making its HTML5 powers more widely known, with the "Brightcove Experience for HTML5", specifically in response to the iPad.
Not to mention sites like TED now offering Flashless renditions for iPhone OS devices. And CBS, the only major network not to be on the Hulu boat, is cleverly testing an HTML5 version of its video site, so even though Hulu won't work on the iPad right now, CBS will be. (I imagine it won't be very long at all before we do see Hulu though.)
It's interesting, to say the least, that a device promising to be the best browsing experience - cue Scott Forestall crazy eyes - is in fact reshaping the internet in some ways. You could argue it's for the better, moving sites away from proprietary formats and heavy, resource-sucking designs to more open standards, and more efficient layouts that are easier to use (as many have, convincingly). And it's not like Apple hasn't been remaking the web already - they've been hugely involved in web standards with their work on the WebKit rendering engine, which powers Safari, Chrome and most every decent mobile browser around. In fact, you could argue, vis-a-vis WebKit Apple's essentially defined the standards for mobile browsing.
There's not much of a choice for site designers to follow this, either. As John Gruber points out, if you care about people on iPhone OS devices - to be clear, that includes the iPad - being able to use your site, you're going to redesign it, and "if you don't think people using iPhone OS devices are an important segment of your intended audience, you're probably wrong."
The reason the iPad could have a more pronounced effect on the internet than the iPhone actually really is simply because it's bigger. The challenge of best displaying your content on the iPhone wasn't simply making sure you had a Flash-less site - it was fitting it all into a 3.5-inch screen, reducing it to the utter essentials to fit the way people use their phones, a task that might've gone beyond a mobile-optimised site in many cases. With the iPad, two of the biggest restrictions - the tighter screen, those smaller windows of time - aren't there, so content producers very well might not need an app to fit their content onto the iPad. In other words, they really can just build a site instead of an app, which is why the iPad might have a more profound effect on the internet than the iPhone.
Especially if Apple manages to sell a bajigagillion of them. But hey, we'll see! In the meantime, there sure are lots of Flashless, HTML5 sites popping up, and we're bound to hear about more.