For the current state of the art in archiving, the standards are set by Plasmon (and HP) with UDO drives and media, and Sony with PDD drives and media. Both use the same basic blue laser technology, in Sony's case Blu-Ray, but with much greater read/write capabilities including much faster speed and reliability. The media comes in 23GB for PDD and 30GB and with UDO2 60GB for Plasmon. Unfortunately, the costs are substantial, with drives starting over $2000 and media just under $50. Interestingly, Verbatim manufactures a UDO media which is Plasmon compatible and lower cost. Here are some links to the technology and reviews of both formats:
Plasmon UDO (Ultra Density Optical) Overview - Archive Without Compromise PC Pro: Product Reviews: Plasmon UDO PC Pro: Product Reviews: Sony BW-RS101
My suggestion as to the best approach would be to utilize the current affordable technology of CD/DVD and hard disk archiving in a redundant process that allows for verification of data integrity and then transferral to new technology in a reasonably short time frame, such as 5 years. I'll bet by then Blu-Ray and HD-DVD will provide cost effective writable drives that provide a viable alternative to the current technology provided by UDO and PDD drives and media.