At the last minute we changed ideas on bathroom trim. We had been planning on using slate tiles cut in half (the same as on the floor) to create a baseboard, but after cutting a few and propping them up we found we didn't care for the look.
Instead, we went and bought a few pine boards to create a nice, simple baseboard. Here's a picture of the (cut, but unstained) baseboard running from the sink to the tub tile:
If you look closely, you will notice that the tub tile and edge only stick out about 3/8 inch. To create a more natural transition between the baseboard and the tile, we cut a 45º angle with our miter chop saw.
We have the wood measured, cut, and dry fit. Now we need to stain it mahogany (the same color as the door in the bathroom) and nail it up. We will likely go with a very similar simple baseboard for the whole house if this turns out well!