And I will love you forever for it. :)
I unintentionally got my little brother into a shitload of bands/artists that I'm a fan of (though, for some reason, he got into Godflesh before I did); he's now got a very similar taste in music to me, though I'm not particularly fond of Woods of Ypres like he is.
Lordnecronus, the exact same thing happened with me and my little sister. I got her into bands like Within Temptation, Sirenia, Theatre of Tragedy, etc. Now when I go back to visit, my C Ds mysteriously go missing a lot.
She still thinks some of my favourites (like Benedictum) are "tuneless screaming" though.
What's the point in giving up when you know you'll never stop anyway?
Oddly, the only time I can think of this happening on a mass level is back when I got all my high school friends listening to Wesley Willis.
In general, I got into "Alternative Rock" in 1994, my sister kinda followed on her own a little bit later, and for a while we had a fairly big effect on each other's tastes just by borrowing cd's. I guess this still happens to a lesser extent now that we're friends on last.fm.
The girl I'm kinda going out with now has pretty mainstream music tastes, including country, so when I ended up making her a mix CD I opted to mainly use songs by folk or country-tinged (or just plain poppy) indie rock bands. She said she really liked it, and I guess that wasn't just talk, because later she'd put up "Home" by Edward Sharpe And The Magnetic Zeros and "I'll Bring The Sun" by Jason Collett on her facebook.
Erm... I guess I've spread some of my taste in music to Zudak and Lucky, and possibly some other people round here? But that's been reciprocated in at least equal quantity.
I did manage to introduce my housemate to both The Mountain Goats and Sufjan Stevens though, and I managed to make a whole room full of people at a house party interested in School Of Seven Bells.
My hetero life mate Xilli X borrows his taste almost exclusively from me. I introduced him to pretty much all of his favorite bands.
As for my girlfriend: nothing. Diddly dick. It's difficult even finding music we can both tolerate (mainly Muse, VNV Nation, and Susumu Hirasawa).