Nion has come out with a great tutorial this week on how to create dispersion and caustic effects in Blender. Although the results are not entirely and physically correct, it makes for a great workaround.
I guess most of the people think dispersion and caustics are not possible to achieve using the internal render engine of Blender 2.5. In my opinion nothing is really impossible – a lot of things are only more complex to set up and/or less physically accurate than with other render engines. Dispersion and caustics are a good example for that.