Yes, they are relatively new. For more on the history of DVD, see this page:
http://www.videohelp.com/dvd
I typically buy double sided DVD-R disks to burn more than 4.7 GB of data. These you flip over when you want more content than can fit on one side of a DVD. Alas you have to split up the content. Depending on the type of content this can be done by segmenting an image, or compressing the data itself and segmenting a compressed file. What's odd is that some disks which appear on the surface to be dual layer because of their single sided capacity, such as the Tiger installation DVD, are in fact readable by all DVD drives that are on Macs, whereas the commercial DVD+R DL is not. I don't know the difference between commercial and consumer grade Dual Layer disks, but there does appear to be one which makes them readable on Commercial grade whereas Consumer is not on older DVD players.