I've been slowly exposing my mum to some of my music. She's really into Pink Floyd and HIM at the moment (The Wall was a big hit), and she enjoyed the first part of Opeth's Lamentations DVD - all the Damnation tunes. I didn't want to push it further with the heavier stuff. Even some of Dream Theater's Live at Budokan went over well.

Now she understands why I freak-out over Mike Portnoy/DT every time they're mentioned on TV.
I should explain: my mom and dad are currently filing for divorce, so she's living with the hubby and I until the divorce is final and she has a nice place to live. We had an extra bedroom and bathroom, so it was a perfect situation for her. Most of the time the television is set to VH1's Classics Channel - it's virtually all music all day in our household. She's always been a huge Beatles fan, and she became familiar with a lot of different groups due to me blasting music in my bedroom constantly whilst growing up, or forcing her to listen to my cassettes in the car. Now she's grown quite interested in KISS, The Who, David Bowie (yay!), and she'll even sit through an entire That Metal Show...show.
I'm sure her fondness for The Beatles and The Rolling Stones has made it a bit easier, as opposed to my dad, who only listened to classical music until he bought a Harley and discovered Meatloaf and tattoos in his late 50's. Late bloomer.