There's a new (and rather expensive) Sony product: http://pro.sony.com/bbsc/ssr/m... that they're saying will be in densities up to 300 GB. From what I've read it looks like a Blu-Ray sized MiniDisc.
I'm not saying these are any better, but there is higher optical densities becoming available. They're targeting high-end video right now in the US markets. I tried to read-up more on this out of curiosity a while back but majority of info in on Sony.jp site site in Japanese. At $6,000 for the drive alone, hopefully there'll be some cheaper alternatives out there. The media prices are much less .. I see one site with a 300 GB disc for $50.
It seems Sony is partnering up with Panasonic. Hopefully something interesting happens with this.
I'd like a backup alternative to DVDs and doesn't seem to be worth the leap to go to Blu-Ray and most tape is too expensive for home use (and/or hard to find).