My first thought is: are you sure the app is connected to the SQL 2005 machine? Perhaps it is using the old connection info or cached connection info and is inserting to the old SQL 2000 server.
What is the insert command?
Can you execute an ad hoc query from the app? If so, have it execute: Select @@ServerName
Are there insert triggers or replication on the table that might be rolling back the transaction after the data is inserted.