I'm using turn.js for a page-flip effect, and it doesn't seem to work very well on IE7/IE8 (load up the website in IE7 mode and you can see for yourself).
What's the best way to support old browsers, while keeping the cool page-flip on modern browsers? (Note: I don't need the hot corners at all on IE, as I also have dedicated arrow links which flip the page via Javascript, similar to the turn.js demo.)
Do you have a sample of how you're applying turn.js? As in, do you have a link or how is your page structured? These are a good place to start.
More the the point of gracefully failing when you detect an older browser: There are multiple options, some make more sense than others depending on how much of the site is already done. First off, how does the site look with Javascript disabled completely? I know this isn't the case for IE7/8, but start there. Can it be used with no JS whatsoever? If the answer is "no," think of ways to make that answer "yes" if you can. This is always a good question when thinking about how to handle older browsers.
If you're early in the site's programming, or the content is laid out in a clear way, then you can work around the limitations of older browsers. A practice I try to follow is designing the site without JS or with as little as absolutely possible, and then program in functionality for "whizzing and banging" afterwards.
In your case, I would suggest something along the lines of: if you detect IE7/8 (or mobile browsers that may not be up to snuff, etc) then load in a secondary stylesheet with rules that give your content as much of the feel of the "original" as you can. Next, have the JS for the older browsers show/hide the pages of content instead of page-flipping them - this can be achieved with some divs on the left and right that move the page and then track the current page vs possible pages (are there any before this page? after this page?) in order to show/hide the navigation divs. turn.js just looks like fancy animations for the same thing, so you should be able to say "Hey, this browser is IE 7 or IE 8 - so, let's not initialize turn.js and just turn control over to the page previous and page next divs."