One good place to start is Microsoft's Note InterOp and collaboration guides.
http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/interopmig.mspx
and http://www.microsoft.com/technet/interopmigration/collaboration/default.mspx
These guides show you the ins and outs of the "free" tools and how to co-exist and migrate from Notes (and other products).
The Collab Team (Erik Ashby and cohorts) now also have a blog to get the latest information about the migration collaboration tools:
http://blogs.technet.com/collabtools/
Another approach is found on Avidware's site:
http://www.avidware.net/migration/lotus/
If it is a larger migration you might want to consider using tools to ease the migration.
One of the remaining tools is the Quest Notes Migrator for Exchange (formerly from Wingra). This automates most of the process for the administrators and reduces the impact on the users.
http://wm.quest.com/products/notesmigratorexchange/
- Joel
I found another reference blog post with some good information in it:
http://blogs.technet.com/eileen_brown/archive/2006/01/27/exchange_and_notes.aspx