Starting with Minecraft version 1.6 (tentatively scheduled for spring 2013), Mojang will no longer be supporting Java version 5 or PowerPC.
Why are you removing support for Java 5 and PowerPC?
Support is being removed so that a number of important updates can be made, and will, among other things, prevent crashing in certain situations, bolster Linux support, and cause Minecraft to run and render faster. Java 5 is no longer being supported by lwjgl (Lightweight Java Game Library; the code library that allows Java to interact with your computer's graphics card), and PowerPC is no longer being supported by Apple.
What is Java?
Java is the coding language that Minecraft is written in. Any program written in Java requires that you have it installed before using that program.
How can I update to the newest version of Java?
If you do not have automatic Java updates enabled (or if you need to download Java for the first time), you can obtain the newest version by visiting http://java.com/download
What is PowerPC?
PowerPC is a type of computer architecture used from 1992-2006, mostly in Apple computers. At this time, PowerPC is typically considered outdated and obsolete.
How can I tell if I have PowerPC?
If you have an Apple computer, you can visit http://support.apple.com/kb/HT1531 for more information.
What should I do if I have Java 5 or PowerPC?
If you have Java 5, you can visit http://java.com/download to update to the newest version. As PowerPC is obsolete, we estimate that this part of the update will affect an incredibly small portion of the Minecraft playerbase. However, if you have a computer with PowerPC, we recommend against updating to Minecraft version 1.6.