Interbase never blocks readers, either (its internal versioning scheme is very, very similar to the Stonebraker scheme implemented in Postgres and they had the same insight that this scheme makes it relatively easy to provide a high level of concurrency).
SleepyCat does some to be good, at least users are enthusiastic about it.
I think they're adding subselects to MySQL, at least they've talked about it. Referential integrity (foreign keys) don't seem to be on their list, however, since they talk about them being a bad idea.
There's no doubt that MySQL is improving, but as Ben notes it is in ways that the implementors have argued for years are "wrong". It will be interesting to watch them change the spin they give to the party line, if nothing else!
