MySQL 4 vs 5 - threading model change

When I ran version 4, mysql would start as many mysqld process as it
needed. This would vary depending upon how busy it was. As a lay
perosn I will refer to this as the threading model.

I have upgraded to version 5 and notice that no matter what it only
runs 1 mysqld. Everything seems to work fine, but I want to make sure
that mysqld is taking advantage of my multiple processors. Has the
htreading model changed and now MYSQL will only run one mysqld OR have
I missed something and need to enable it?

Travis