1. Locking is done at the storage engine level. So, for MyISAM, I believe that the answer would be Yes.
2. Partitioning in MySQL 5.1 doesn't support parallelism. (We're planning to add this, but it probably won't happen in 5.1.) Partition pruning can be quite fast, and I believe that in at least some cases, it can apply to multiple partitions, which is still faster than scanning an entire table. (See
http://dev.mysql.com/doc/refman/5.1/en/partitioning-pruning.html for an example.)
3. That depends. If you're adding new, empty partitions or reorganising existing partitions that contain relatively few rows, it's quite fast. If you're reorganising existing partitions such a way that it's necessary to move a great many rows into new partitions, this could take a while.
Jon Stephens
MySQL Documentation Team @ Oracle
MySQL Dev Zone
MySQL Server Documentation
Oracle