There was a pretty successful Kickstarter a while back for a product ("Coffee Joulies") that was exactly that, but for your coffee mug. Little bean-shapes made of stainless steel, filled with material that changed phase at 140F.
There's also Ember [1] (also a kickstarter I think) that just made a mug with a heating element and controller to keep your beverage at a specific temperature. Using materials with specific phase change temperatures sounds like an innovative solution, but it's also grossly engineering a problem that can be solved better by $10 worth of components.
Is having a processor and electronics and software and sensors, which, despite the cost of components of $10, requires electricity. Is that really that much better than just building a better thermos?
I don't know, personally I would absolutely hate having to remember to keep my mug charged - although it's a damn cool product, just not for me.
I'm not sure I can agree about a phase-change solution being overengineered, however; while the materials would be more complex, the design itself should be far simpler than an electronic mug, and exactly the same as a vacuum sealed one.
I'd also be really worried about toxicity and materials that changed phase at that temp... Or well any temp. Can it leech through the walls? I wouldn't be Keen on dropping molten steel in my coffee either.
Seems clear that the stainless steel is to keep the material out of your liquids. It is not molten! Keep in mind you regularly put stainless steel in your mouth and lick it.
