the 5.1 RC release is Heikki's scaling patch for autoincrements on
multiprocessor machines. In versions previous to the 5.1 RC Innodb
could have issues with load if you had multiple CPU's competing with
threads over who got to assign autoincrements.
The following graph shows the change in a test where mysqlslap was
used to generate a write test. In this case a specific load was
applied as a single user and then the load was split among many users.

The second graph shows a "scaling" test where each client adds the
same load as the last. As you can see this also makes a significant
difference in performance.

As load increases in the second graph, Innodb scales linearly. It is
great to see the Innodb development team working on performance (and
Oracle's commitment to continuing development on Innodb).
Even if you are not interested in any of the features present in 5.1
but you use Innodb, you should check out the latest RC.
http://dev.mysql.com/downloads/mysql/5.1.html