So I guess in the latest Spore patch (1.05) they added support for exporting the creature models as Collada (.dae) files. Unfortunately it's only for Windows at the moment but I think we're expecting the Mac release any day now.
It took the better part of an afternoon to figure this out so I thought I'd share my knowledge to make it a bit easier for you.
Exporting the Collada model:
Go into spore and load your favorite creature in the creature creator.
When you're in the paint tab hit ctrl+shift+c. This will bring up some sort of cheat window. Then just type in 'colladaexport' (omit the quotes) and press enter. You'll be presented with a EULA that you have to agree to and it will save the model in My Docuements/My Spore Creations/Creatures.
Converting .dae to .3ds or .obj:
You can't just import the .dae directly into blender (I think because blender can't handle Collada 1.4.1?). I found a great tutorial on how to get around this problem using a free Autodesk converter (scroll down… the stand-alone converter is in the middle of the page).
Load the .dae in the left hand pane. Then before doing anything else hit convert. This will give you a .fbx file with the same name. Now load that .fbx file in the left hand pane (remove the .dae). Change the destination format to either .3ds or .obj.
A note on formats:
You can export .3ds without changing any of the settings.
If you export .obj you need to un-tick both boxes ("triangulate" and "bake deformations").
Import into Blender:
Now just go File->Import->(.obj) or (.3ds)
For .obj have "NGons as FGons", "Lines as Edges", and "Keep Vert Order" selected.
For .3ds the default should work.
Slice and Dice
This is the tricky part. I myself am having problems with a bunch of the models slicing in skeinforge (i keep getting "ZeroDivisionError: float division").
I'm thinking the problem has to do with the object not being manifold; though I've tried going into edit mode, hitting ctrl+shift+alt+'m' to select holes, and using 'f' to fill them with no success. I need a Blender expert to get me out of this bind.
Here are some of my files:
The original Collada file: Beasty Boy.dae
A working STL file: Beasty_Boy_working.stl
A not working STL file (I'm pretty sure I'm using the same process I did to get the working one). : Beasty_Boy_notworking.stl