I’m not sure how aggressively I’ll start going about it, but I’m pretty sure DITA-OT 1.6 will be the last version to have Java 5.0 as the minimum Java version; Java 5.0 has been End of Life’d since Oct 2009. Java 6 doesn’t have that many features which I would like to immediately use in DITA-OT, but e.g.
javax.xml.stream.XMLStreamWriter would be good.
The DITA-OT usage survey seems to support the move, only one participant claims to use 5.0, but even that entry has also marked 6 in use. (If you didn’t participate in the survey and you absolutely need 5.0, this is the time.)