You really cannot talk about warranty in such a general manner. You have to look at the actual one. Very few things affect the warranty, either something is covered, or it is not. With the N900, the only things that voids warranty (but not neccesarily so), is to physically fix it yourself (take it apart), do something with the IMEI number or throw it in the toilet, in the microwave owen or some similar abuse.

In addition there exists laws in each country, that will blow most of the warrany statements of the N900 out of the water. This is also stated in the warranty of the N900.

Rule of thumb is that you can do whatever you like, except those things that actually voids warranty. But these things can be different from product to product.