  
      if hasattr(sys, 'real_prefix'):
          prefixes.append(sys.real_prefix)
-@@ -1162,6 +1152,8 @@
+@@ -1078,6 +1068,8 @@ def change_prefix(filename, dst_prefix):
              if src_prefix != os.sep: # sys.prefix == "/"
                  assert relpath[0] == os.sep
                  relpath = relpath[1:]

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):

             return join(dst_prefix, relpath)
     assert False, "Filename %s does not start with any of these prefixes: %s" % \
         (filename, prefixes)
@@ -1190,6 +1182,11 @@ def install_python(home_dir, lib_dir, inc_dir, bin_dir, site_packages, clear, sy
     site_filename_dst = change_prefix(site_filename, home_dir)
     site_dir = os.path.dirname(site_filename_dst)
     writefile(site_filename_dst, SITE_PY)
+    wrapper_path = join(prefix, "lib", py_version, "site-packages")
+    writefile(
+        join(site_dir, 'sitecustomize.py',),
+        "import sys; sys.path.append('%s')" % wrapper_path
+    )
     writefile(join(site_dir, 'orig-prefix.txt'), prefix)
     site_packages_filename = join(site_dir, 'no-global-site-packages.txt')
     if not site_packages:
