I know some people have done it, and I've considered it but haven't taken the plunge yet. Mostly because I'm trying to change the color of the gauges, not necessarily make them brighter. If you're looking for better lighting, replacing them with LEDs would be a good idea. But if you're looking to change the colors, it'd be rather difficult as the gauges are already coated with that teal/green color.
Anyway, these LEDs are a direct replacement for your bulbs..
T1.5 LED Twist Lock Lamp Specifications