In a mailinglist I read that doing a lavalamp seems to be a problem relevant enough to do a tutorial for this. I tried myself - not using any tutorial - an found out the problem being the actual "lava". I took a particle system spreading out meta-objects and in one rendering I accidently swithced the particles to faces. Adding a cylinder I got a tree inside a lavalamp.

The glass is a raytraced material. The floor is from an image (I think from the 3DSMax CD, but I'm not sure), the cube's granite is from the net and the wall is made using Daniel Yu's Brick Texture plugin.