George, the batteries in question are within the pressurized fuselage, so ambient temperature is not an issue. And should the aircraft lose pressure, the passengers and crew are at much greater risk of dying than those batteries.
The choice of battery type is an engineering decision based on a number of factors, primarily cost and weight. Of course, mass reduction is an overriding concern in aircraft design, second only to safety. Lithium ion offers the highest energy density of the commercially available battery technologies, so it's hardly surprising that Boeing went with li-ion. My guess it was a global decision: "every battery in the aircraft must be li-ion". I don't think the batteries in question are that large, so they could have gone with a different technology for those particular batteries with a small weight penalty.
I doubt Boeing will switch to a different technology. More likely they will go with a modified battery or charging system. Switching technologies would require modifications to the charging system, so there's little to be saved in terms of cost by switching to NiMH.
With regards to your Prius, a hybrid like the Prius doesn't need a huge amount of storage capacity. Ni-MH makes a lot of sense, since it offers very good energy density, is reasonably cheap and reliable. On the other hand, a plug-in EV or hybrid like the Volt needs a lot of storage capacity. That's why the Volt uses li-ion batteries.