What I was expecting to happen was:

With a full cauldron, I wanted to extract a source block of water with a bucket and end up with a bucket of water.

What really happened was:

The bucket remained empty.

Other commentary

This is something that Dinnerbone has asserted is intended behavior. See also:

https://twitter.com/Dinnerbone/status/270246317849190400

I'm adding this report mainly to keep it on the radar as an open bug, as it hasn't been fixed (contrary to previous assertions). The assumption is that this is a way to obtain a water source block from rain on challenging survival maps.