Our built-in GE microwave died last week and after doing some research, we decided to replace it with a new Panasonic microwave. The main reason for the decision was the Inverter technology employed by Panasonic. Of course, every microwave that's built-in requires a different this hole and has its own trim. Unfortunately, the requirements for the old one were larger than for the new one, even though the interior size of the new one is bigger.
After doing some research I found this company: http://www.microtrim.com/microwaves.htm . They make custom trim kits for microwaves, ovens and cooktops! I found a couple of reviews by others who were extremely positive and the cost of them is lower than the factory ones! Ordered mine today and will be here next week.
Not something everyone needs immediately, but certainly something to keep in the back of your mind for the future.