While this will work, it is not a recommended configuration.

Two reasons.

- You can only restore WITH NORECOVERY, hence you can't use the logshipping secondary for reporting

- If you ever fail over to the secondary (the primary fails), it's a one-way failover and you will NOT be able to fail back to the primary without upgrading that to SQL 2008 R2 first. Hence any DR usage is essentially a permanent move of the production DB.

Since log shipping is usually used for reporting or failover in the case of a disaster, a up-version log shipping is a rather poor idea since it works for neither.