According to the Wikipedia article on 3DS Max (which may or may not be accurate) plugins written for 7 are compatible with 8, but not 9, 2008, or 2009.
Which version of 3DSMAX the plugin is written for is still one of the outstanding questions, but I would imagine that with the source, the plugin could be ported/upgraded/downgraded to other versions of 3DSMAX with a little effort. Of course, I've been wrong before, so we'll have to wait and see.
If the information provided by Paradox in
this thread is correct, it may prove difficult to recompile the plugin.
On that note, does anyone here own or have access to a copy of the 2009 SDK? I only have access to a copy of Design 2009, unfortunately.