If you have a look through some of the Google datacenter tours you'll see they actually implement this idea on a massive scale, though I believe they use lead-acid batteries... Every motherboard in the datacenter has a small battery capable of powering it for a long enough period of time for the backup generators to kick on and take over.

It's definitely a clever idea! Makes maintenance much less risky as it's distributed, and also reduces the chance of having a single large UPS fail when it takes over the load!