Currently they're only available in monthly and annual versions, but as the price drops they will become available for weekly and daily tickets too. Conveniently, they come just when I'm about to buy my first monthly travelcard for my new job, so I can be a cool trendsetter with my nifty remotely-read smartcard, which will almost certainly fail to work in a humourous fashion the first time I use it.
Forgive me for being geeky here; I don't really know why I find these so cool. I've just been waiting to see these cards introduced for a long time -- the little yellow readers started appearing on tube gates more than a year ago. And I really do hate the horribly low-tech paper tickets, they're such a pain.