Depends on what level of x86 you're wanting to emulate. I have a copy of RealPC Classic (or maybe it's SoftPC Classic? I can never keep it straight), which emulates a 8088-level PC on a Macintosh Classic. I've successfully run Windows 3.0 in real mode on it, for certain values of run. There are also versions of SoftWindows that run on 68k; I'm pretty sure I got Windows 3.1 going on one once but can't remember. It's been a long time.
Probably the craziest thing I've done recently is get Mac OS X running on a 68k Mac via PearPC running in 68k Linux. Took literally days to boot.