Martin Fowler on In-Memory Databases and Testing

Just came across Martin Fowler's post on In-Memory Databases. I was glad to see his mention of Firebird, which has been a favorite of mine for a number of years. Fowler talks about the primary use of in-memory databases: testing, or to be more precise test-driven development, where speed is of the essence. He also mentions SQLite, which I've only recently started playing with, mainly by going through the Django tutorial. I'd like to explore it further though in the context of testing.