A woman with silicone breast implants (primarily), among other silicone implants associated with plastic surgery. A Silicone Sally is a play on the trophy wives of the millionaires living in Silicon Valley (alternatively called "Silicone" Valley to satirize these women).
Dude #1 "I met a Silicone Sally with the biggest fake titties ever last night when I was clubbin' in the valley!"
Dude #2 "Did you get a feel?"
Dude #1 "No, she was with her husband."
Dude #2 "Then how did you know they were fake?"