The Photobooth app on my shiny new iPad2 (yeah, get over it ;-) has a "Thermal Camera" setting, so I snapped this. I'm not entirely convinced how accurate this is, but it does point to something very interesting - the existence of thermal microclimates within a vivarium.
When your frogs wander around, they're doing more than entertaining themselves, they're regulating their body temperature by seeking out the correct thermal zone. This is one reason why frogs will always be happier in a big vivarium than a small one (in addition to all the other benefits).
As the weather gets warmer - be careful, don't let them overheat!