Use of the GPL will be optional, as it can still be used under the CDDL. According to the NetBeans FAQ the reasoning to release NetBeans under a dual license is that, "The GPL v2 license will provide an additional option to vendors that are unable to work with NetBeans software under the CDDL license. Adding GPLv2 as a license option will make NetBeans software even more Linux friendly. Adding GPLv2 with Classpath exception to NetBeans software will keep product portfolios and bundles consistent. Sun open sourced its JDK implementation under GPLv2 and the GlassFish project is dual-licensed under CDDL and GPLv2 with Classpath exception."
Bruno Souza, the NetBeans Community Manager for Sun had also earlier said, "As you all know, we are working hard for the release of NetBeans 6.0. The new release will bring exciting technical features and this is a great time to consider what else can we do to empower the NetBeans Community. As a result of requests from the comunity, we are considering the potential adoption of a new license model. We are considering releasing a future early access version of NetBeans 6.0 under a dual licensing scheme of CDDL and GPL v2 with Classpath exception. A move like this would be well received by many of the NetBeans contributors, and will benefit the community at large."