I find of the most exciting tidbits about scalability from reading the mailing lists of popular data stores that people are ramping up to huge deployments (MongoDB and Cassandra primarily, but Redis and CouchDB occasionally).

All those people with multi-TB systems scaled across many data centers inevitably end up on those lists asking great questions about exactly the types of problems you run into at that level.

It helps give you perspective before you are the one with a geographically disperse collection of terabytes of data; which can be a godsend.