I’m hearing theoretical longevity for BluRay tests are close to 100 years. I understand Delkin has archival media that tests to 200 years.
At some point in the near future Sony will offer a firmware upgrade for the PDW-U1 XDCAM drive to allow it to be used as a data file store. XDCAM disc is considered good long term storage given the case.
I do not trust hard drives in general but double back up to optical disc should be a good fail safe. Drobo certainly looks to be a good back up system but you’ll have to keep buying more (and keeping them active) as you shoot more material.
With the MxR solution you can buy a Transcend 16GB SDHC card for about $30. While not cheap it’s certainly not worse than HDCAM tape and it may even start to approach the price of XDCAM discs in some months.