In an interview with Newsarama, Allison Mack (Smallville's Chloe Sullivan) was asked whether or not she would do a cameo on Supernatural:
Nrama: I know you have to go, but everyone wants to know if you'd do a Supernatural cameo. Any chance?
Mack: Ah! [laughs] I'd love to work with Jensen (Ackles) and Jared (Padalecki)! That would be great fun!
Source: Newsarama.com.