Does the XNA render pipeline have built in occlusion culling?  I'm running the multitextured terrain tutorial from Riemer's, no water though, and it's running pretty slowly.  One model, a sky box, and a map that's 600 by 600 "tiles", each 100 units square, and it's running at 14 FPS.  I wrote an occlusion method to test the bounding box of each tile against the camera frustum and make a new index buffer based on the included verts, and it actually ran slower.  I did notice that with none of my own routines though, the frame rate sped up when I was viewing less terrain, which makes me think it does in fact have built in occlusion. 

 So, does it support built in culling or do I need to write a better algorithm, and does anyone have any tips to speed it up a bit?

There is no built in culling in the low level XNA graphics API. I don't know about Riemer's tutorial.

Creating new index buffers each frame is likely to be very slow: you generally don't want to be modifying GPU resources dynamically. The best way to do this kind of thing is to pre-sort your buffers into sectors, so you can just issue draw calls specifying different sub-regions of the existing index and vertex buffers depending on your culling results.