After seeing MakerBlock's blog post yesterday I was inspired. So I went down to Walmart and bought 2500 4.5mm BBs (for $4) and set to work printing my own ball bearings.
I first tried printing the inner part in two pieces, but they proved hard to glue together. So I tried printing the inner part as one piece. I didn't think I'd be able to get enough BBs into the channel for it to work. I found that with pliers I can just muscle the last two or three BBs into the channel. The plastic deforms a bit, but it returns to normal immediately.
With this design, it makes me wonder if some parts that are designed to fit a 608 bearing could just have the outer bearing feature included. The you just print the inner part, add the BBs and grease and go!