Hidden costs of open source

The following letter was printed in ITWEEK, in response to this article: "Hidden costs of open source" by Martin Butler.
The points Martin Butler makes are accurate, but only in respect to open source software products like MySQL. MySQL uses a dual licensing policy, a commercial license that allows the MySQL database to be distributed with your application, without having to open source your own code, and a free software license if you are distributing your own open source applications that comply with GPL terms.This is pretty much the norm for most commercially viable open source projects, but not always. For example both Firebird - a relational database based on the open sourcing of Borland's InterBase, 4 years ago - and PostgreSQL do not use GPL as their license.So both can be freely deployed with commercial applications without requiring a license fee, or open sourcing your application code. Firebird uses IPL, the InterBase Public License, avariant of MPL, whilst PostgreSQL uses the BSD license. Paul Beach