I would like to add a ramp to my map that can flatten and rise when a button is pressed/unpressed. Since I'm going for the clean, Wheatley Laboratories theme, I would like to use panels to do this. However, after spending far too long in the model viewer, I still cannot find a model with an animation that involves rotating 45 degrees (to form the ramp). Is there a model that does this that I missed, or is there another way of making a ramp that can be switched on and off?

http://developer.valvesoftware.com/wiki/Creating_rotating_panels should help

The arm64x64 also has a set of animations called "rampa_stair01_up" and so forth.

That would probably be better than a rotating brush.

I've used them vigorously in my maps.

I've used two tactics in my map. The arm64x64 does have the rampa_stair0x animations which, when the panel are put in the middle of a brush twice their width (so a quarter of the brush is on either side) it makes a nearly smooth ramp.

The other option is using anim_wp/arm_interior_192/video_arm_big.mdl which is a arm that has an animation to move to a 45 degree ramp from flat, however, it's a specialized animation and sort of "shakes" first, but will do what you want.

Either method works, it depends on which you prefer. I use both in my map, if you want to go have a look. Good luck!