We are in the midst of a MySQL 5.1 to MySQL 5.5 upgrade. Nobody upgrades the database engine on a running production system without a Darn Good Reason, but we had recently run into one with a bug in MySQL 5.1 that was exploding MySQL RAM use on our production server and threatening to crash the production database.
Ultimately, the problem was we wanted to use "statement batching" with UPDATE, which produces a large number of UPDATE statements in a single long string, dramatically improving performance. After a dialog with MySQL's QA people, it turned out that we were hitting a bug in 5.1's query parser in which a lot of RAM was allocated for every statement, and not freed until the entire set of queries in the query text was parsed. So, this wasn't, strictly speaking, a "memory leak", but still an open-ended memory allocation which caused the RAM assigned to MySQL to grow enormously.
5.5 appears to have solved this problem, and we're well along in our acceptance process. Other than a couple of very small issues with syntax that was deprecated in 5.1, but now an actual error in 5.5, all is going well. We'll go live shortly.