You get a 1 yr manufacturer warranty (from Nokia). If you have a credit card in the USA your credit card may offer an additional year warranty if you buy the product with the credit card (E.g. my VISA card gives me an additional 1 yr warranty) so I am now covered for 2 years.

The SquareTrade warranty, has to be bought off Amazon (for $50) but this requires you to buy the n900 from Amazon. This one is for 3 years. You can buy the warranty from SquareTrade directly but it will cost more than $50 (though not now with holiday discounts) and will cover you less (2 yrs instead of 3). These warranties start the moment you buy the device so in reality your only covered for the two years after the first year's manufacture warranty expires.

For SquareTrade you need to send them your invoice of wherever you purchased the the device.