This really isn't about invalidating the warranty, it is about what is covered and what is not on Nokias behalf. Only deleting/removing IMEI and taking the phone apart will (possibly) invalidate the warranty. Besides, there are consumer laws that usuall are much more consumer oriented than these warranties, and laws cannot be omitted by warranties or agreements.

But make no mistake, damage on the phone, casued by software or any other thing, software or hardware not authorized by Nokia, is not covered by Nokias warranty. It is written in the last pages of the manual (or rather the quick start guide). This is of cource totally out of sync with how the N900 is marketed, and Nokia should change this, as this is illegal marketing or an illegal restriction on the warranty. For instance this site:
http://maemo.nokia.com/maemo/
where they talk about the benefts of open source software.

The only possible conclusion is that maemo.org is in fact authorized by Nokia. But what about other open source communities??