Hold it! The competition is advertising on *my* list now? Hah, never mind, CL-RDBMS is a nice system, but to answer your question about Postmodern: Yes, it is thread-safe, as long as you do not have two threads use the same connection object at the same time. Using WITH-CONNECTION, it is easy to give each thread its own connection. The only part of the library that uses global mutable state is the connection pool, and I used bordeaux-threads' mutexes to prevent problems there.
Cheers, Marijn