I am on MacOSX Lion with EagleCAD v6.4.0
Ah. For some reason, the MacOS version was never able to import files as old as the windows version.
I recently did the exercise of doing Eagle6/MicroXP/VirtualBox and manually updating all of my ancient schematics, just in case.
MicroXP/Eagle6 in one window, Mac EAGLE7 in another. Sharing the Eagle Project folder. Try to open the file in the Mac version; if it fails, open in the Windows version instead, make a trivial change, save...
It worked fine. A PITA, but I didn't have quite enough old files to work on automating the process at all, especially given that Cadsoft wouldn't tell me how to detect old file versions... It might have been worse if I had any designs that exceeded the limits of the freeware version.
(MicroXP is pretty neat, and I'm pleased that all old versions of Eagle seem to run on it.)