This is probably one of those no-brainers, but until now I wasn't certain whether one needed to manually erase a DVD-RW or DVD+RW rewritable disc that already contains content. In iDVD 4, you could write to rewritable media by inserting a DVD-R after you click iDVD's Burn button, then sneakily ejecting it and inserting a DVD-RW disc. But, you had to use the Finder or an application like Toast to erase the disc first.
iDVD 5, however, recognizes when a content-containing disc is in the SuperDrive and asks if you'd like it to be erased first. After clicking Yes, the normal burn process begins; after all the data has been encoded, iDVD performs the actual erasing, and then burns the new data to the disc.
Like I said, it makes sense, but I hadn't seen it mentioned anywhere yet.