This problem may also be due to caliper plate lift; which happens when the stainless steel brake pad guide reacts with the alluminium alloy of the caliper and corrosion sets in. The corrosoin displaces the stainless steel guides, forcing the pads against the disk causing brake squeal. It also makes pads difficult to remove and often when fitting new pads the edges have to be ground down.
This is a common problem with RS2 porsche calipers, both standard and big reds. Newer porsche monoblocs such as the 996tt and GT3 calipers do not have this problem as they do not have the stainless steel brake pad guides. Often the squeal happens as when you are not using the brakes, when you apply the brakes the squeal goes away
The stainless steel guides can be removed (not always easilly) and the corrosion can be filled back as per this link:
http://www.arthurlea.com/Porsche/Brakes/brakes.htm
Or if the corrosion gets too bad you can get new calipers and if you don't want it to happen again get monoblocs.
other reasons for squeal could be build ups of brake dust, dirt and/or old grease or copper slip, pushing the pads against the disk. You could try giving the calipers a good clean, renewing the shims, springs etc and refitting them dry, as in without any grease (as I think recommended by porsche).
HTH
Angus