According to the Eclipse FAQ. I read that
Eclipse DOES NOT consult the JAVA_HOME environment variable.
Eclipse does not write entries to the Windows registry
As far as I know, to invoke the JVM, Eclipse executes the java command, so, if it's at your system execution path, Eclipse won't need to find the instalation directory.
To check, you can start a OS shell and execute the next command (from any location):
> java -version
EDIT: I was (partially) wrong. From the FAQ (http://wiki.eclipse.org/FAQ_How_do_I_run_Eclipse%3F#Find_the_JVM)
If a JVM is installed in the eclipse/jre directory, Eclipse will use it; otherwise the launcher will consult the eclipse.ini file and the system path variable
So, Eclipse first looks inside its eclipse/jre dir for installed VMs, if empty, consults the eclipse.ini file, and at last, looks at the system path.