Fire up your Software Update, because there's a Java update waiting for both Snow Leopard and Leopard users. For Snow Leopard (requires 10.6.3), there's Java for Mac OS X 10.6 Update 2, which weighs in at around 80 MB. For Leopard users (requires 10.5.8), there's Java for Mac OS X 10.5 Update 7, a much larger 122 MB download.
Both updates bring Java SE 6 to version 1.6.0_20 (compatible only with Intel-based Macs), while the Leopard-only update also updates Java SE 5 (compatible with Intel and PowerPC Macs) to 1.5.0_24. The upshot of these updates? "Improved reliability, security, and compatibility" of Java in OS X, with more detailed security notes available for the truly curious.