Just a postscript from my post in another thread.
OK gang, this problem has been around for over a year, it's a hardware issue, and Apple knows about it. The solution is simple, but not even Apple (via their so-called Genius Bar) seems to know it. You can get Verbatim "Digital Movie" DVD-R's at Best Buy for $17/25 that Toast will burn at 8x. In my opinion $.68 is a great price, considering you're actually getting great quality media.
The drive used in the recent iMac's and laptops are very picky about the quality of the media. This is not a conspiracy by Apple to sell blank disks even though Apple branded DVD-R's ($35/25) will burn at 8x. There are solid technical reasons why the good Verbatim disks will work as well as the Apple disks - they're made by the same people, at least the ones sold in the US. See http://www.digitalfaq.com/media/dvdmedia.htm for the details. The bottom line is Mitsubishi makes the best blank DVD's right now. Apple buys their present supply from Mitsubishi and all of the Verbatim disks sold in the US are made by Mitsubishi. The other brands could be made by any of a number of different manufacturers, some great - some lousy.
BTW, I've seen this question so often this year in various forums and forms, that I think that this should be in the Toast FAQ.