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!
The trick is to get your tolerances correct. My Makerbot prints holes slightly undersized, so I increased the diameter of the inner hole a little. Also, getting the size of the channel correct is key if you don't want a sloppy bearing.
I print these parts 4 at a time using the multiply feature. The outer part really doesn't need much work, but the inner parts need some cleaning because the channel surface ends up a little bit messy when the nozzle goes from one part to the next. Some sandpaper did the trick for me.
Adding the BBs isn't too difficult. Just twist the inner part slightly and add as many BBs as you can. I get 6 or 7 into the channel by hand. Then, for the last two or three BBs, I use channel-lock pliers or vice grip pliers to just shove the BBs into the channel. Make sure to wait for the plastic to completely cool first, otherwise the deformation might be permanent.
I added a bit of grease and am actually amazed how smooth and tight the bearing is. Obviously it isn't sealed so I don't know how long it will last, but it seems to work great.