For whatever reason, all the medium-to-large arboreal lizards I have kept (a young male iguana, two female water dragons and one male water dragon) have had an intense liking for sitting on top of computer monitors. Whenever I've let one roam free, the lizard would typically start out by patrolling our territory for interesting insects to meet and kill (actually this only applies to the dragons, iguanas are vegetarian), then tease me for a while, then it'd hop up on its preferred spot -- on top of the monitor. Even the most curious and active water dragons I've kept has liked to sit and lazily perch on the monitor for hours, if I let them. I can live with that -- a real, live "monitor lizard" looks so much more amusing than those beanie babies some people like to keep on top of their monitors. It's also a bit funny, because there's a genus of lizard named monitors (the Komodo dragon, Nile monitor and the Australian Goanna are examples -- not that water dragons or iguanas are related to those horrors). From what I've read on reptile pages on the 'net and what I've heard from other reptile-keeping freaks, this is actually pretty common lizard behaviour.
The obvious explanation for why lizards like monitors is the heat output. CRT's get hot, and the topmost plastic surface makes an excellent spot for a little reptile to rest for a while, lie flat and soak up the heat. All exothermic animals need external heat sources for their metabolism, so it's safe to bet that nature has wired their little brains so that a good heat source feels pleasant to them -- in fact, one of the tricks I've used to tame lizards is to help them realize that sitting on my arm is an good source of heat in the morning (the lizard quickly realizes that it won't be hurt even when sitting on that big strange human -- this builds trust). A monitor is better though, so if I bring my water dragon with me over to the computer, she'll leap up to her preferred spot on top of the monitor within a minute.
It could also be because the ancient reptile vs. mammal feud isn't over yet. That would explain why even the most well-behaved of my little "monitor lizards" will always find the most annoying spot on the screen to dangle its tail in front of. I'm sure that when LCDs finally overtake CRTs, lizards will enter an unlikely alliance with cats in hating computers.