The solution is to hook up the mouse to a Windows system (or use Boot Camp) and install the Logitech Setpoint software for Windows. After the software is installed, simply open the gaming mouse application and set the preset for each of the buttons to the 'generic button' setting. Save your settings and unplug the mouse. Now, when you hook it up to your Mac, all buttons should now be visible to Steermouse and USB Overdrive, as well as the Expose and Spaces Preference panes, allowing you to assign them in any way you please.
This same technique may be effective with non-gaming mice as well, but I have not tried it (Logitech has different configuration applications for gaming and non-gaming mice).
[crarko adds: I haven't tested this one.]

