I checked with Philip support today (August 2nd 2007) on the status of the SPD7000BD. Here is what they told me:
"The drive is able to read a Dual Layer disc, but it is only capable of writing to the single layer Blu-Ray disc. The hardware is not able to burn to a Dual layer 50GB disc, so no firmware update will enable that feature."
This seems fairly clearly.
Sigh. I had bought my drive under the impression it would support 50 GB backups, but it looks like I need to buy some more 25 GB disks instead...