As previously covered by InfoQ, Apple's involvement with the OpenJDK project began in earnest in November 2010. The first fruits of this labour became available last week, with the first commit and build available to run OpenJDK on OSX.
The current status builds a JDK bundle (as understood by the new layouts in OSX) as well as hotspot support, networking and X11. Not only that, you can already run Eclipse on OSX thanks to the cross-platform SWT support being bound to the native code already.
Since some of the graphical aspects of Apple's JDK implementation were significantly different from the way that other platforms are implemented, there are some missing parts in that regard at the moment. This means that the WebStart, Applet and the Java preferences aren't part of OpenJDK on OSX yet; nor are some of the other aspects like clipboard and printing.
Running Java headless applications (or SWT-based Eclipse applications) are likely to work well and gives OSX a breath of life in the JDK space. As this depends on 10.6, the OpenJDK package only supports Intel processors; PPC processors will not be supported. The current build requires a 64-bit processor to compile and build the package, although the compiled result will run on either a 64-bit or 32-bit platform.
There is a community supported Google code project which provides pre-built versions of the OSX OpenJDK tree; so a compiled installer for JDK 7 build 121 is available.