While those docs are not released by Oracle, a good start is improving ( or gaining ) skiils on Cluster, ASM, RAC and Linux. At least when You get those DOCs, You'll be well prepared and understanding Exadata Will be easy ( or less hard ) ;)
Along with RAC, ASM, and Oracle Architecture, it helps learning about storage, and infiniband. Also look out for features like Hybrid Columnar Compression, Storage Indexes, Smart Scan etc.
You can also read some good blogs post about database machine on Pythian blog at http://www.pythian.com/news/