The problem is seen when running the pbxt.pbxt_xa test case:

pbxt.pbxt_xa w1 [ fail ] Found warnings/errors in server log file!
Test ended at 2010-05-10 08:54:14
line
safe_mutex: Found wrong usage of mutex 'LOCK_sync' and 'LOCK_active'
^ Found warnings in /home/knielsen/devel/maria/my/work-5.1-security-fix/mysql-test/var/1/log/mysqld.1.err

This is not a new bug, it just became visible after fix of Bug 544173 (before
this code path just crashed the server).

Until this is fixed, I will add code to pbxt_xa.test to suppress this warning.
As part of fixing this bug, please make sure to remove this suppression.