Real time rendering is already available on todays graphics workstations. I know XSI will allow you to do this. I'm not familiar enough with other modeling/rendering software enough to know which other software will allow for real time rendering.

You can have fully rendered scenes in real time using techniques such as Refraction, reflection, global illumination, etc... It's all very impressive and gives an incredible time advantage to artists trying to get the scene just right.

There is also a custom display host in XSI that should allow you to hook the display engine into XSI so that you could get immediate feedback on how your models look in the game engine. BSP calculations will not be real time. So it's not going to be perfect until Octree is available.

EDIT: After I posted this, I thought I would try to find the resources I had seen that told me about the realtime rendering through XSI. I thought I had seen them on the NVIDIA web site. But I wasn't able to find it again. I did find an article about tight integration of XSI with the Nvidia graphics workstation cards.