I've been trying to find a good source for what the planets look like in the infrared, specifically if viewed as a point source. I've been working with a low resolution telescope that senses in the infrared. What will a planet look like?
A planet's IR appearance depends on several factors including
If you're observing with broadband filters and simply imaging, unless the planet has a complex cloud structure you should expect the planet to behave more or less like a black body at the mean surface temperature of the object.
Using narrowband filters or low resolution spectroscopy, you start to move more and more away from the ideal blackbody in appearance. Especially if there is a complex cloud system on the planet (like on the Jovian planets). In this situation, you have a couple of effects.
Finally, as your spectroscopy goes to higher and higher resolution, you will be able to resolve these effects more and more.
Here's a picuture of the Earth's spectrum (Taken from this page.): A wave number of 1000 cm-1 corresponds to a wavelength of 10 microns. As you can see it generally follows the shape of a blackbody but there are several absorption bands due to the presence of various elements in the atmosphere.
As the planets get bigger and more complex the spectrum differs even more from a black body to the point where what you have is peaks of emission in various bands that are coming from deep in the planet and huge absorptions features from the clouds higher up.
According to the Wikipedia article on black bodies, the "Earth in fact radiates not quite as a perfect black body in the infrared", but it appears to be close.
To recover this answer from the massive edit, this is how the Earth looks in near infrared:
Here's Jupiter, infrared on the left, visible on the right (source).
Here's Saturn (source).
And Uranus (source).
And Neptune (source).
(Sorry these aren't point sources.)