Remember how the gap between Minecraft 1.2 and 1.3 was several months? Prepare to wait again, because Minecraft 1.6 won't be here for quite some time. Nathan Adams tweeted today, "Come Minecraft 1.6 we'll be dropping support for Java 5 (and old PowerPCs). It won't be for a few months, but go update early and save time!"What does he mean about dropping Java 5? Well, Minecraft is built apon the Java framework (that's why you need Java to use Minecraft). Every version of Java since 1.0 has added some new features for developers to take advantage of, and Java 5 is old. Java 5 is from 2004, and most computers have at least Java 6 and some have 7. The only common computers that are stuck on Java 5 are all Macintosh computers made before 2006 (aka PowerPC Macs). If you are running at least Windows XP, you are good to go.
Source: Twitter
Update: In a response to a user by Nathan, he clarified that they will drop support, not remove it. So it may work, but with serious bugs. In other words, if you update to 1.6 on a PowerPC Mac and the game is unplayable, it isn't Mojang's problem.