How worried should I be about head bumps as my infant learns to get around?
Suggestions:
Toddlers happen to be "designed" with the right height and mass that they very rarely injure themselves when falling from their own height. Mostly what you need to worry about is them falling from higher than their own height, like from furniture or stairs, falling onto something sharp, or something heavy falling on them. Most of the falls you are worrying about now, you will notice shortly that your child looks to see if you're watching before deciding whether it's worth it to cry or not. My 18 month-old actually head butts things for fun now.

My 7 year-old has cerebral palsy from her skull getting a little squished during her premature birth, so we are well acquainted with the signs of brain trauma. Mostly what you need to look out for is a sudden change in typical behavior, like:

Sluggishness
Eyes looking glazed over
Seizing
Vomiting
Dizziness
Slurred speech
Non responsiveness
Being inconsolable after several minutes
If those things happen after a nasty fall, consult a doctor right away, or even get an ambulance depending on severity. Bumps and bruises are not fun, but are normal and nothing to worry about. They happen even to children of the most conscientious parents.