Yesterday I gave OOo 3.0.0rc1 a try. For no apparent reason, the developers of OOo 3.x changed the folder structure of the OOo installation (at least on Windows XP). You have now inside your OOo installation folder two subfolders: OpenOffice.org and OpenOffice.org 3. All of OOo's JAR files are located in subfolders of OpenOffice.org: Basis 3.0\program\classes and URE\java. soffice.exe is located in a subfolder of OpenOffice.org 3: program.
With this new folder structure the Bootstrap class in the JAR file juh.jar (now located in OpenOffice.org\URE\java), cannot locate soffice.exe anymore! Bootstrap only looks in the current folder and in the parent folder of the juh.jar file location for soffice.exe. So it seems, it's not that clever to separate them too far from each other.
So if someone encounters "no office executable found!" with OOo 3.0.0rc1 solve it this way for example: Copy the juh.jar from OpenOffice.org\URE\java to OpenOffice.org 3\program and put the juh.jar in OpenOffice.org 3\program into your java CLASSPATH. Or give [Java solution] "no office executable found!": http://user.services.openoffice.org/en/forum/viewtopic.php?f=44&t=2520 a try.
I'll further investigate this problem with the next versions of OOo 3.x. If the OOo developers don't change this, OOo forums might get flooded with "no office executable found!" posts...