In the gimp Dicom plugin, I solved this by calculating the minimum and maximum gray levels and then rescaling these to 8-bit. Very primitive, but if someone just wants to paste an MRI-image of their brain into a larger image it is enough.
A more sophisticated approach is my image viewer, giv, that supports various image pixel depths, including floating point and 32 bit images. Giv has a Contrast tool that allows interactively seeing the histogram and choosing part of it to be displayed in the image. Search for Dicom in the giv manual at http://giv.sourceforge.net/giv/giv.html to see an example. Giv allows plugins so if all you are looking for is to view the image, that might be enough.
(If someone (yes, me included) would feel board on a rainy day, I think it would be cool to add the giv contrast tool to the gimp dicom plugin to allow playing around with the gray levels before committing yourself and choosing a certain window for import. This could be a general way of dealing with high-contrast images).