As those of you who are using OneNote 2007 know the file format in OneNote 2007 is completely different from ON2003.  That being said David has a great post: Why the OneNote 2007 and 2003 file format are different.  Some interesting tidbits:

Impact of this difference on interactions between OneNote 2007 and 2003 clients

  • OneNote 2007 can open OneNote 2003 files read only. This is the default. In this case the format will not be changed and the files will still be editable and viewable by another OneNote 2003 client, and they can be viewed in OneNote 2007 but not modified.
  • OneNote 2007 can UPGRADE OneNote 2003 files. The user is prompted and asked before doing this and it is explained that OneNote 2003 clients will no longer be able to open them. In this case, the file is now fully editable by all 2007 clients but no 2003 clients.

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We understand this issue will cause inconvenience for some, and trust me we did not take this decision lightly. It was a very difficult and painful decision. We do think the improvements in OneNote 2007 are pretty dramatic and compelling and were worth the change. Thankfully many of our beta users seem to agree. Also the upgrade pricing for OneNote is reasonable, so given the compelling improvements we expect as most people upgrade (particularly those who want to share with other machines), this will quickly become less of an issue.

On a final note, rest assured that we are very committed to file format compatibility as a goal for future OneNote versions.