.US opened April 24, 2002 (with that initial 2-year registration requirement). The drop process (approx 6 weeks on hold, then 30 days in RGP, then 5 in Pending Delete) would push those drops back to early July 2004. Might have a few drop that were registered before the public rollout but I would think most of those would have been good enough that people would want to keep.
Of course, A fair number of .us are already dropping because the registry started allowing one-year regitrations in Sept 2002. So a lot of those names started falling in late November 2003. Hardly any that are any good though since they were all registered 6 months or more from the initial rollout.