Hi C..
I do not come from the school of thought that this is not a big deal and you should just let it pass. Biting is always serious, however how you deal with it can make it into a HUGE problem, or it can end it quickly.
If you hurt your child back when they do this (by biting, flicking the cheek, etc.) you may get your child to understand that when they bite, they will get hurt back, so they stop (cause and effect). What they also learn eventually is that it is okay for someone bigger than them to hurt them, but it is not okay for them to hurt others. Cognitively, they are in a very egocentric phase. They cannot yet understand how their actions effect another. I understand how traumatizing this can be for you, your child and the "victim," especially if your child is in day care. Consistency is very important in how it is handled.
Biting occurs for many reason. Young children are very oral, some more so than others. They explore their environment through their mouth (taste and touch). So redirect and give them something that is okay to bite, explore with their mouth, and of course explain to them, "Ouch! Stop! Biting hurts! You can bite this instead." Again, they won't neccessarily understand this completely yet, but they will eventually. And you are teaching your child to be respectful by respecting where they are at developmentally.
Sometimes children bite as a sign of affection! They get so happy to see you that they just bite. Again tell them that it hurts and to stop, and give them another way to show affection. Say, "Be gentle," as you gently stroak her face. Tell her what is okay more often than you tell her what not to do.
Sometimes kids bite because they just have so much energy. They have to learn constructive ways to expell that energy in an appropriate manner.
Sometimes, and likely for your baby's age, they are teething. Give her something she can chew on.
Sometime's children need to learn how to deal with being frustrated or angry. She may be a bit young for this yet, but you can always give her the language (sign works great for some!) she needs, ie: "help please!" or "milk" or "eat!".
Depending on how you have been reacting, she could be playing the cause and effect game too. Again, teach ~ gentley ~ and redirect, and be consistent.
Like some mentioned, pay attention to her cues. What is going on with your child before the biting occurs? You will be able to pick up on the reason pretty quickly.
Having worked in child care for a number of years and raising 3 of my own, I have seen biting gone VERY bad for people that did things such as bite back. I certainly don't mean any disrespect to those who have chosen that route, for I realize we all do our best.
Make sure that if there are young bitees, they learn how to react appropriately as well. They need to be empowered to say, "Stop!".
Also, keep in mind that at 8 months old, your child is not malicious. They do get excited about cause/effect! Respect her. Teach. Redirect. Love her, which of course you already do! And remember that children learn what they live. The Golden Rule is not just to be taught to children, it is for us to remember too.
Best wishes to you and your precious baby girl!
H.