I see that SP5 now depends on concurrent.jar (that wasn't the case before). The usage is only in 2 places now: TransactionImple.java for the core and jts packages.
My question, however, is why did you choose to go with an old and unsupported concurrent.jar instead of going with backport-util-concurrent?
The latter superseded the concurrent.jar, got all the development on, and is actively maintained. My only guess is that would be for JBoss AS compatibility, but what about bugfixes? BUC also have features backported from Java 6, which is not the case for concurrent.jar.
Yes it's for AS compatibility. If you want to use a more recent alternative just change the import statements and recompile the source.