Royans Tharakan writes about scalable web architecture, covering NoSQL, Google's Pregel and Spanner, and migration stories such as Wordnik and Reddit.
HN links it for summaries of large-scale systems, including Google's Pregel and Wordnik's move from MySQL to MongoDB, and Reddit's scaling troubles.
Google has published a very short paper about a program, called Pregel, which expresses graph algorithms using message passing between vertices. Apparently its scaling doesn't suck:
http://www.royans.net/arch/pregel-googles-other-data-processing-infrastructure/
They do try to group related vertices as much as possible, but if edges span computers, that's okay too. I assume that it works best when the interconnect between computers is low-latency, though.
http://www.royans.net/arch/pregel-googles-other-data-processing-infrastructure/
http://blog.udanax.org/2009/08/inference-anatomy-of-google-pregel.html
http://portal.acm.org/citation.cfm?id=1073981
also check out PIs publications and his thesis http://research.google.com/pubs/author145.html
The lead author will also be giving a presentation on Pregel at SIGMOD 2010: http://www.sigmod2010.org/research_list.shtml
I think Reddit will be doing something like that. Jereny from Reddit responded to the thread and mentioned they might be moving to cassandra. Its however not a trivial task for a site with that kind of load and data to cutover :) http://www.royans.net/arch/reddit-learning-from-mistakes/comment-page-1/#comment-534
Video of this talk is available here: http://blip.tv/file/3621377
This article also has some details: http://www.royans.net/arch/mongodb-migration-from-mysql-at-wordnik/