In Mountain Lion how do I set, in one place, the PATH environment variable for all newly launched applications?
After many trials with different options I conclude:

Is there a unified way to set the PATH for applications launched by different means (like shell script, Finder and Spotlight)? Well, yes and no:


No: Setting PATH in shell start up scripts (e.g. .bashrc) works only for apps launched by shells.
No: Setting PATH for the current launchd process via launchctl setenv PATH &lt;...&gt; does not work for me at all.
Yes: Setting PATH in /etc/launchd.conf (and restarting the system) will make the appropriate PATH visible in  applications launched by Spotlight, Dock and Finder and subsequently (local) shells. 
No: Remotely invoked shells do not inherit the environment. (See ...why do remote shells via ssh do NOT inherit the environment..) 


Notes for set-up of shells:


For correct functioning in local shells it is important to switch off the use of /usr/libexec/path_helper, because the default paths in /etc/paths might change the order of the paths in PATH.**
To accommodate remote shells you need to re-import the launchd environment into the shell. Use something like this in your .bashrc:


# the vexatious path stuff
eval $(launchctl export)


Unfortunate consequences of the whole issue are:


No way to have these unified settings on a per user basis. (~/.launchd.conf does not work.)
No easy way to construct the PATH from different sources using techniques like home directory expansion et al.