This bug report was two problems:
1) LAST_INSERT_ID() returns a value which does not exist in the table
2) the reporter would want it to return the autoinc id of the updated
row.
1) is a real bug, 2) is a feature request.
In July I implemented 2) in 5.1 (which automatically fixes 1).
This has not yet been documented or released, so is changeable.
Precisely, recently Paul and a user found an easy workaround to give
2), which works in 4.1-5.0-5.1. So I can revert my code for 2),
because it's not needed, that's what I do here;
we forget about 2) (we will document the workaround).
But when I revert my code for 2), 1) comes back. We solve 1) by saying
that if INSERT ON DUPLICATE KEY UPDATE updates a row, it's like a
regular UPDATE: LAST_INSERT_ID() should not be affected (instead of
returning a non-existent value).
So note: no behaviour change compared to the last released 5.1; just
a bugfix for 1).