Avinash Lakshman, Prashant Malik, and Karthik Ranganathan presented a talk at SIGMOD 2008, "Cassandra: Structured Storage System over a P2P Network", that describes a Google Bigtable-like data store used actively by Facebook.
What strikes me as remarkable about the system is its rather casual treatment of writes. As far as I can tell, a write is only sent to memory on one box, not written to disk, not even written to multiple replicas in memory. That seems fine for log or click data, but, for the kind of data Facebook deals with, it seems a little surprising to not see a requirement for multiple replicas to get the write in memory before the app is told that the write succeeded.
The code for Cassandra is open source and there is a wiki that adds a few tidbits to the SIGMOD slides. Note that HBase is also open source and also is modeled after Google's Bigtable; HBase is layered on top of Hadoop and sponsored heavily by Yahoo.
Please see also James Hamilton, who posts a summary of the slides and brief commentary, and Dare Obasanjo, who offers more detailed commentary on Cassandra.
Please see also my post, "Highly available distributed hash storage from Amazon", about Amazon's Dynamo (PDF). There are similarities in the design and references in the wiki that suggest that Facebook's Cassandra was influenced by Amazon's Dynamo.
If you are interested in all things Bigtable, you might also enjoy Phil Bernstein's post, "Google Megastore", that summarizes a Google talk at SIGMOD 2008 on a storage system they built on top of Bigtable that adds transitions and additional indexes, among other things.
Update: Avinash Lakshman swings by in the comments and clarifies that Cassandra does have a commit log, so writes do go to disk on at least one machine immediately. A write first updates the commit log, then updates the memory tables, and, finally, in batch some time later, goes out to all the tables on disk.