I'm simulating a cloth. How can I bake the cloth simulation into keyframed vertices or shape keyed vertices?
If you look in the Modifiers section of the properties you will see an Apply button (you probably don't want that one) and an Apply as Shape Key button. That's the one you want.
After you have turned the cloth shape into a Shape Key, you will have to go to the mesh section and dial up the influence of the new Cloth shape key to 1.0 to change the cloth into the shape that you "baked".
If you want a single frame, see Mutant Bob's answer. If you want the entire cache, select your cloth object and export it as an .mdd or .pc2. You can then bring this cache back in using the Mesh Cache modifier, or you can import it as a series of shapekeys (1 for each frame.)
There is an alternate way which seemed easier for me, as I only needed 2 shapes. (I tried some other methods first.) 1. Simulate your cloth and bake as desired. 2. Duplicate the cloth mesh, move it slightly above. Remove cloth simulation, shape keys, ... to get just a plain mesh. 3. On the timeline, go to the frame you want for the cloth shape. 4. Select the duplicate cloth and add a Shrinkwrap modifier. Shrink to the original with the shape. For example, "project negative Z". Fine-tune until it looks right. 5. Apply the Shrinkwrap modifier as a shape key to the copy. It will be removed from the stack. You can test it by turning up the influence of the shape key. 6. On the timeline, go to another frame with a cloth shape you want. 7. Repeat 4 and 5 as many times as needed.