Minecraft wiki entry for Development_versions, associated text: Added a notice/warning for users still playing on PowerPCs or Java 5 (Java 1.5).
Mojang plans to drop support for Java 5 by version 1.6.[5] This is due to the most recent versions of LWJGL no longer supporting Java 5, with PowerPC also no longer supported by Apple to be made compatible with Java 6 or newer.[6]