There is a version for Solaris and other unix platforms to test native motif applications. It is called QAPartner though. Same language as SilkTest (SilkTest used to be called QAPartner). The unix versions can't test browsers though.
Okay, related, but different question. Has anyone ever tried to run SilkTest on a Linux box via WINE? I was just pondering yesterday if this was possible and thought about giving it a try in my copious free time.