I tried installing 2.1.2 from the MacOS .mpkg and get the error below. Same or similar issues 
have happened in past:

http://mercurial.selenic.com/bts/issue3120
http://mercurial.selenic.com/bts/issue2879

I have toggled VERSIONER_PYTHON_PREFER_32_BIT and ARCHFLAGS to no avail.

Tried installing with pip to no avail.

One thing: I believe this is the first time I've tried installing Mercurial since updating to 
Xcode 4.3.2.

~ $ hg
Traceback (most recent call last):
  File "/usr/local/bin/hg", line 38, in <module>
    mercurial.dispatch.run()
  File "/Library/Python/2.7/site-packages/mercurial/dispatch.py", line 27, in run
    sys.exit((dispatch(request(sys.argv[1:])) or 0) & 255)
  File "/Library/Python/2.7/site-packages/mercurial/dispatch.py", line 40, in dispatch
    req.ui = uimod.ui()
  File "/Library/Python/2.7/site-packages/mercurial/ui.py", line 43, in __init__
    for f in scmutil.rcpath():
  File "/Library/Python/2.7/site-packages/mercurial/scmutil.py", line 421, in rcpath
    _rcpath = osrcpath()
  File "/Library/Python/2.7/site-packages/mercurial/scmutil.py", line 393, in osrcpath
    path = systemrcpath()
  File "/Library/Python/2.7/site-packages/mercurial/scmutil.py", line 442, in systemrcpath
    path.extend(rcfiles(os.path.join(p, 'etc/mercurial')))
  File "/Library/Python/2.7/site-packages/mercurial/scmutil.py", line 431, in rcfiles
    for f, kind in osutil.listdir(rcdir)
  File "/Library/Python/2.7/site-packages/mercurial/demandimport.py", line 86, in 
__getattribute__
    self._load()
  File "/Library/Python/2.7/site-packages/mercurial/demandimport.py", line 58, in _load
    mod = _origimport(head, globals, locals)
ImportError: dlopen(/Library/Python/2.7/site-packages/mercurial/osutil.so, 2): no suitable 
image found.  Did find:
	/Library/Python/2.7/site-packages/mercurial/osutil.so: mach-o, but wrong architecture