I've googled around, but I can't find any valid codes - just discounts on particular devices (e.g. 5800), and nothing for the n900. It might become easier to get a discount on the n900 once it has been released for while.

If you want it within the next few weeks other retailers will probably be the cheapest option. MPD have it for 450 sim free, but there might other cheaper ones.

Apparently you can get it for free if you are willing to sign up to a 24 month contract.