Janis Joplin
I heard a rumor that Zooey Deschanel might play the "Queen of Rock and Soul".
She can not play Janis Joplin. If she is going to play a woman of the psychedelic rock era, she should play Grace Slick. She has those large "striking" eyes and pale skin. That, and having actually heard her sing, I feel that she would sing 'White Rabbit' beautifully. Her voice is just deep enough to pull it off.
As for Janis, leave her alone. If you can't find the right one, you don't have to settle. Physically, you need a woman with...
-A round face
-Eyes that squint when she smiles
-Little teeth + wide smile
-Slightly below average-sized chest
-Slightly above average-sized butt and thighs
My suggestion, Joss Stone. I am not a Joss Stone fan, I find her songs incredibly annoying. However, I have heard her sing 'Piece of my Heart', it was not a perfect match but it was quite good. That and having seen her on "The Tudors" as Anne of Cleves, I must admit I was quite impressed by her acting abilities.
I believe (at least by comparison to the many other actresses rumored to play her), that she would do a wonderful job of playing Janis Joplin.