After some discussion on the Apache Karaf dev list we've come to a consensus on Java 1.5 support. The Karaf 2.x line will continue to maintain JVM 1.5 compatibility. Going forward, with the Karaf 3.x line, JVM 1.6 becomes the minimum supported JVM version.
As of this writing no 3.x branch of Karaf exists. We currently only have 2.1.x and 2.2.x series releases planned which will of course be JVM 1.5 compatible. Any future 2.x branches will also be JVM 1.5 compatible.