Sun has updated the wikis.sun.com section for DTrace to include very detailed documentation. The wikis micro-community at sun.com is still in beta. You can login and make changes. Once logged in, there are different permissions for Sun employee and non-employees.
There is also a DTrace section on the "Solaris Internals" site, solarisinternals.com and of course there is a "DTrace Users Guide" on docs.sun.com.