It's not just TiVo locking down their hardware
In the few months I've been working at the FSF, I've been dealing with about twenty license violations on GNU software. When it comes to the embedded devices, I've noticed an upsetting trend: a lot of them are locked down. The products range from wireless routers to personal media players. They take different strategies, too; some of them won't provide you with the tools necessary to build your own firmware, and other times the hardware checks the software for authorized signatures. The result is the same either way -- they'll give you the source, but you can't actually modify the software you're running.
This is a real problem that affects free software users today, and it's only going to get worse if we don't fight it every way we can. Right now, if you don't like the firmware the manufacturer provides, you can often install an alternative, like Rockbox or DD-WRT. But there are products on the shelves right now that will prevent that, and more on the way.
This is the reason the GPLv3 drafts require that these distributors make it possible for you to modify the software it includes and install those changes. We didn't add that just to stop TiVo, and we're not looking to force our will on the Linux kernel. This issue affects the entire free software community, and all the devices we use. We need to turn the tide.