If a query does a SELECT the optimizer will discover which partitions
that has to be scanned. In 5.1 these scans will still be made
sequentially on one partition at a time. However only those partitions
actually touched will be scanned which can improve performance on
certain queries by a magnitude.
The aim is to support parallel scan in a future release as it is to
support parallel sort on the first table selected by the optimizer.
This is an important long-term goal of partitioning to open up the
MySQL architecture for many performance improvements on
handling large data sizes. In 5.1 we have achieved quite a few of
those goals but expect to see more goals achieved as new versions
of MySQL hits the street burning :)