This is true on some systems, but not others. On FreeBSD (and OSX, as I remember) /dev/random and /dev/urandom act identically. The underlying CSPRNG, Yarrow,

This is true on some systems, but not others. On FreeBSD (and OSX, as I remember) /dev/random and /dev/urandom act identically. The underlying CSPRNG, Yarrow, is designed to recover from a compromised state vector. Unfortunately, Yarrow relies on entropy estimates to time its re-seeding. I've said it before: entropy estimates are a fiction. Ideally, /dev/random and /dev/urandom would both offer non-blocking access to an underlying Fortuna implementation (using a cryptographic hash function instead of a cipher, to avoid export/import restrictions).