The idea behind http://www.gigawatts.nl/co/projects/...bel/switch.jpg
is the same as the PC PSUs: you close a connection and it works on
115Vac, you open it and plug the PSU into 230V.

And if you go as in http://www.idesignz.org/misc/Simple%...tage%20PSU.pdf
you're right that the full-wave center-tap mode does not make full use
of the transformer copper (by the way, just adding an electrolytic on
the center-tap in place of the switch you can have the two output
voltages simultaneously). It's unavoidable, unless wou have two
independent secondaries, of course: put both in series or both in
parallel.

But if you are looking for having two different output voltages (not
same DC voltage with 115 or 230 mains), and want to make the best use
of your electrolytics (specially if they are big ones), you should
need a means to put them in parallel for the low setting, and series
for the high. I did it with a DPDT non-shortcircuiting switch. No
center-tap (becomes a full-wave doubler as in the gigawatts link). If
you are interested I will gladly send it to your address, but you can
help me and the Group if you can suggest a convenient way to post
files somewhere (not a volatile place such as rapidshare).