A /dev/random interface for Solaris first appeared as part of the unbundled SUNWski package in Solaris 7. /dev/random in SUNWski is actually implemented as a named pipe which was written to by a daemon process. A named pipe made sense because it was all done in user land. Starting from Solaris 9, /dev/random and /dev/urandom became device nodes since a kernel-based implementation was done. This is available as a patch on Solaris 8 also (112438-03 for SPARC and 112439-02 for X86).
In Solaris 10, /dev/random supports hardware-based random number generators (RNG). It does so by using the kernel cryptographic framework (kCF). One cool thing about this feature is that existing applications which use /dev/random can get the random numbers from a hardware RNG *without* needing to be modified. A hardware RNG has to be registered with the kCF and implement random number generation routines to be usable by /dev/random. For more details about the kCF interfaces, see http://www.sun.com/bigadmin/features/articles/crypt_framework.html or send an email to solaris-crypto-api@sun.com.
Another Solaris 10 enhancement was to make /dev/urandom scale much better on a multi-processor machine. We get near linear scaling for reads on /dev/urandom.