We're in a similar situation, switching from Novell to Server 2008. Our setup is very basic; all we will be using under 2008 is file services (and of course AD and all that entails).
As of now I haven't found anything that will let me migrate the file server role directly from NDS to 2008. If I don't find any such tool, I will probably just use robocopy to copy all the file server data to the 2008 file server ... and then use robocopy daily to do "incremental" copies leading up to the day of the actual cut-over. All while creating global groups etc manually on the 2008 AD.
More info on Robocopy here:
http://technet.microsoft.com/en-us/magazine/2006.11.utilityspotlight.aspx
We do have the advantage of having relatively few users and groups, otherwise this manual approach wouldn't really be feasible.