Without this patch `virtualenv --python=python2.7 .` fails with an error because it notices that the python readline.so is not in the same path as python2.7. I assume this is to avoid copying the wrong file on systems where it is possible to find incompatible libraries by accident. Adding "/nix/store" to the prefix fixes this problem. Unfortunately readline is still not available if you just run `virtualenv .`.

--- virtualenv-1.8.4/virtualenv.py        2013-01-16 23:43:37.583615220 +0100
+++ virtualenv-1.8.4/virtualenv.py        2013-01-16 23:44:47.885973431 +0100
@@ -1135,17 +1135,7 @@
 
 
 def change_prefix(filename, dst_prefix):
-    prefixes = [sys.prefix]
-
-    if is_darwin:
-        prefixes.extend((
-            os.path.join("/Library/Python", sys.version[:3], "site-packages"),
-            os.path.join(sys.prefix, "Extras", "lib", "python"),
-            os.path.join("~", "Library", "Python", sys.version[:3], "site-packages"),
-            # Python 2.6 no-frameworks
-            os.path.join("~", ".local", "lib","python", sys.version[:3], "site-packages"),
-            # System Python 2.7 on OSX Mountain Lion
-            os.path.join("~", "Library", "Python", sys.version[:3], "lib", "python", "site-packages")))
+    prefixes = ["/nix/store", sys.prefix]
 
     if hasattr(sys, 'real_prefix'):
         prefixes.append(sys.real_prefix)
@@ -1162,6 +1152,8 @@
             if src_prefix != os.sep: # sys.prefix == "/"
                 assert relpath[0] == os.sep
                 relpath = relpath[1:]
+                if src_prefix == "/nix/store":
+                    relpath = "/".join(relpath.split("/")[1:])
             return join(dst_prefix, relpath)
     assert False, "Filename %s does not start with any of these prefixes: %s" % \
         (filename, prefixes)