Our 4 1/2 month old Riley does the same thing. It drives me crazy! He can be so loving and then turn around and start nipping at your toes and ankles. He loves fingers too! My husband thinks he does it when he wants attention. It can be VERY painful. We tried saying "Ouch!, No bite!" and ignoring, but that didn't seem to work. I did some research online and decided to try a spray bottle. It does seem to work, but it seems it's not always around when we need it. Especially since he took it and hid it under the bed.
(He's a kleptomaniac!) I would love to hear any ideas about how others are dealing with this problem.