IMHO 500 euros is reasonable considering that this is almost a computer in your pocket with real control for the user. It is my guess that an iphone 3GS would cost more than this without a contract and the device looks good as the iphone.
One reason might be that the Samsung has 8 GB storage, the Nokia has 32 GB. The Samsung has 320x480 pixels, the Nokia has 800x480. The Nokia also has OpenGL for Exposé-like task switching etc.
Rumors and FCC documents suggest that the N900 will be available in USA with T-Mobile's 3G frequencies. Nokia probably wouldn't go through the effort of building a special version for T-Mobile US unless they have a deal in place that the device will be subsidised by the carrier.
So there's a fair chance that this will be available in the US for considerably less than 500 euros (my guess for the T-M price would be $199, but who knows?).
Remember, this is what you pay without a contract. I don't think it's much for a phone (or rather, a full handheld Linux computer) with these specs (32 GB storage, 800x480 display, 5MP camera...). If you prefer to pay it monthly in your contract with a network operator, let's hope there'll be good deals.
