Unfortunately it seems Philips has had a change of plans on 50 GB support.
I just received a SPD7000BD and found it only supported 25 GB writes, even with the latest firmware upgrade.
I checked with Philips support if there would be a firmware upgrade for 50 GB and 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."
Alas. I had bought this drive specifically because I had thought it would support dual-layer Blu-ray writing. But it looks like that isn't going to happen.