(Digital Video Disk Recordable)
DVDRs are massively more efficient as data stores than CDRs. Despite being the same size, the DVD stores 4.7 Gb against the CDR's humble 500-750 Mb.
Which, to us, means that we can't begin to burn any data onto the DVD until we have 4.7 Gb queued for storage. In the early days, when we go live and no-one really understands what we're about, we would be surprised if we collected that much data in less than 3 months. A year later, the rate of upload might increase to one DVD every few days. 3 years down the line it could be a DVD every day or so.
There is no way we can allow such a huge opportunity for illicit amendment. Data uploads must be burned on virtually in real time, in order to minimise the risk of attack while they're held in cache. At the moment CDRs are the only economic option because they support packet writing and can thus be recorded incrementally in small chunks until the disk is full. Standard hardware allows us to write as little as a single sector (2k) which means we can burn every upload as it happens. Clever software might even let us record byte by byte.
In any case, DVDRs won't be that flexible until the next generation. Until that happens, or some completely new WORM technology arises, CDRs will remain our storage medium of choice.