Anyone else received an "Your upcoming Genius Bar appointment" email confirmation that never actually booked an appointment?

If you didn't make the appointment, it is possible someone mistakenly used your Apple ID to schedule an appointment.

It is also possible it's a scam, and if the email is requesting you sign in with your Apple ID and password, it is certainly a scam. In that case, just delete the email and forget you ever saw it.

The following Apple Support documents may be worth reading:

Identifying fraudulent "phishing" email

Identifying legitimate emails from the iTunes Store

There is no reason to conclude your Apple credentials have been stolen simply by having received the email, but if you are concerned you can easily change your Apple ID password. Read the following:

Apple ID: Changing your password

and

iCloud: Change your iCloud account password

I also had that happen... and did what John suggested - changed my password (as well as my apple id).