There are plenty of ways to void the warranty on your new tablet or smartphone. But according to the Free Software Foundation Europe (FSFE), “rooting” it (gaining super-user rights so that you can change any of the software in it) and even changing the operating system for another one are not among the ways to render yourself without a warranty.
FSFE assert that, under Europe-wide consumer regulations required by Directive 1999/44/CE, simply changing the software within your device - although it may be forbidden under the terms of extended warranties offered in addition to your statutory rights - does not remove the manufacturer’s warranty obligations under the law.
So while you might not be able to get software support for a rooted device, any defect that manifests during the statutory warranty period - the first two years of ownership - has to be addressed by the vendor.
A fundamental freedom of open source is to be able to modify your software to better suit your needs. But if you then can’t actually use that software on your device without significant loss of rights, your freedom is still being limited. This legal advice from FSFE suggests you can go ahead and try alternative software on your phone and tablet, safe in the knowledge your supplier still has to honour the warranty on the hardware itself.
Ubuntu Linux, is working hard to get Ubuntu available for the Google Nexus 7 tablet, in place of Android. They seem to be making good progress; here’s a video showing it running with an antique IBM keyboard: