I also wrote a paper about the many ways to set trace in Oracle for the current session or another session and also how to set the different levels of trace. This paper is called "many ways to set Oracle trace for your session, others sessions and at instance level".
The post of interest to me in the oracle-l thread was one by Tanel Poder where he goes into great detail about how X$ tables work and how the MODULE and ACTION data for each session are accessed through the X$ tables. For those amongst you who like internals details. This thread is a must read. The thread by Tanel is here.