my Mac (OSX 10.7.5) has both Java6 (1.6.0_65 from Apple) and JRE7 (1.7.0_45 from Oracle) installed.
Standard is still the Apple Java6 and the Oracle Java7 installed itself in Directory "Library/Internet Plug-ins/JavaAppletPlugin.plugin/Contents/Home" (Yes, there is a blank in the Path Name!).
I tried to set Java Home in sqldeveloper.conf to Java7, but that change does not help starling SQLDeveloper.
Debugged the startup script "sqldeveloper.sh" in "/Applications/SQLDeveloper.app/Contents/MacOS" - it calls the Apple binary /usr/libexec/java_home which does not know of Oracle Java. After skipping this check manually the dependent scripts seem to have problems with the Blank in my path (quotes around env. variables missing). Now I'm stuck ...
My Mac OS should be certified and my Java configuration is pretty standard - is this a bug to be fixed? Meanwhile I stick with SQLdeveloper 3.x