The 512GB SSD that Apple is offering as a build to order option is the Toshiba H2, according to Accelerate Your Mac.
A reader (thanks Bradley) sent links to photos of the 512GB SSD (option) in the new MacBook Pros (here and here) which shows it's made by Toshiba and the part number ID's it as an H2 series.
Recent benchmarks show that the SSD is outperformed by most other SSD offerings; however, it does offer double the capacity.
I wouldn't touch one of these. There are plenty of other options that a so much faster and cheaper. As for capacity, stick to SSDs for a boot drive not your porn collection.