Honestly, most kids can me rather philosophical about things. She probably already knows. I mean, she's 10, she goes to school, and since she's asking questions, she's probably smarter than you've given her credit for.
If you up and tell her, will she be angry? Maybe. But how would you feel if someone lied to you for years and then when you started to get suspicious finally told you the truth? Granted, most people separate regular lies from make-believe, but kids don't always understand the difference. However, at 10 she should be able to.
Here's what I would recommend. Sit her down and tell her--and when you do, explain why you did it (because of how much fun it is, etc). If she gets angry, apologize as sincerely as you can. She'll get over it.
Personally, my mom told me that Santa was just a man that dressed up, that it was all make-believe, from the time I was old enough to understand the concept of pretend, and I never had any heartbreak about it. As much fun as it is to believe in Santa, that is what I will do with my daughter. It's just easier--and then the kids don't feel like you've betrayed them.