The room demo in this link: http://away3d.com/away3d-23-feature-overloadhandles collision by using a collision map or image. Works pretty well if you collision system is actually 2D.
I've made a couple of first person attempts using away3d + jiglibflash and the results were excellent. This is if your collision system definitely needs to be 3D. The bounding box of collision active meshes are related to physics boxes. I did experience little problems on the player warping through the floor on a 2 floor level. The ground is impenetrable but the boxes (used for walls generally) aren't. Collision precision depends on cpu stress, so if the cpu is on the edge, the player can start warping through things. I hear there is a FP10 jiglibflash version on the making, which combined with away3dlite should put the stress limit further from your needs.
