Pete Finnigan has mentioned my blog in his Oracle Security weblog, especially the security-related post: How to see the MOD_PLSQL passwords in clear text
Thanks to him for his comments!
I want to note that this information can be found at his book: Oracle Security Step-by-Step (Version 2.0) as step 8.1.2 from Phase 8 - Application Servers and the Middle Tier.
Additional information online:
Fact sheet about Oracle Mod_PLSQL passwords
How to Unencrypt the Portal Password in Wdbsvr.app