Inkscape's current version, 0.48.2, will open Illustrator 8 files with a plugin called UniConvertor, which is has binaries to install on Windows and Linux, but not on Mac. It may be possible to install it via the python installer provided with the source download, but there are a lot of dependency issues and I gave up on this after an hour or so. It is available from http://sk1project.org/modules.php?name=Products&product=uniconvertor .

However, Inkscape is currently working on a native Mac version which includes UniConvertor, or at least supports the .ai v8 file type. There is a development version available, see http://www.inkscapeforum.com/viewtopic.php?f=5&t=13610&p=52928#p52909 . I'm sure that the main Inkscape site will announce when this has passed all the regression tests and is considered ready for general use, but I have installed the 0.48.4 development version and found it usable now (it's even a native Cocoa app, doesn't require X11, which is fantastic). At the very least, I can open .ai files exported from MoI, save them as .svg files, and open those in my older Inkscape version.