Issue with 'asv preview' on MacOS X

I'm not sure if this is a MacOS X-specific issue, but when doing ``asv preview`` on MacOS X, I get:

```
Traceback (most recent call last):
  File "/Volumes/Raptor/Library/Python/2.7/bin/asv", line 9, in <module>
    load_entry_point('asv==0.1', 'console_scripts', 'asv')()
  File "/Volumes/Raptor/Library/Python/2.7/lib/python/site-packages/asv-0.1-py2.7.egg/asv/main.py", line 22, in main
    args.func(args)
  File "/Volumes/Raptor/Library/Python/2.7/lib/python/site-packages/asv-0.1-py2.7.egg/asv/commands/preview.py", line 32, in run_from_args
    return cls.run(conf=Config.load(args.config), port=args.port)
  File "/Volumes/Raptor/Library/Python/2.7/lib/python/site-packages/asv-0.1-py2.7.egg/asv/commands/preview.py", line 44, in run
    httpd = MyTCPServer(("", port), Handler)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 419, in __init__
    self.server_bind()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/SocketServer.py", line 430, in server_bind
    self.socket.bind(self.server_address)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
socket.error: [Errno 48] Address already in use
```

I checked, and localhost:8080 doesn't work (and no IPython notebooks are running). Any idea what could be causing this?