I have seen this issue at several customers, when migrating from Notes (Domino) to Exchange 2010 using the Quest Notes Migrator for Exchange (Quest NME).
The problem is ActiveSync not working correct on Nokia phones. Only new calendar items (after the migration) is synced to the Nokia phones. All older and therefore migrated calendar items are not synchronized to the Nokia phones.
I have now come across a great script from Kent, which corrects the calendar items.
Read his blog post and get the script here.