diff --git a/dmemcache.inc b/dmemcache.inc
index 6a68b14..b7a2128 100644
--- a/dmemcache.inc
+++ b/dmemcache.inc
@@ -418,20 +418,8 @@ function dmemcache_object($bin = NULL, $flush = FALSE) {
 }
 
 function dmemcache_key($key = '', $bin = 'cache', $reset = FALSE) {
-  static $prefix;
-  if (!isset($prefix)) {
-    $prefix = '';
-    // memcache_key_prefix can be set in settings.php to support site namespaces
-    // in a multisite environment.
-    if ($prefix = variable_get('memcache_key_prefix', '')) {
-      $prefix .= '-';
-    }
-    // When simpletest is running, emulate the simpletest database prefix here
-    // to avoid the child site setting cache entries in the parent site.
-    if (isset($GLOBALS['drupal_test_info']['test_run_id'])) {
-      $prefix .= $GLOBALS['drupal_test_info']['test_run_id'];
-    }
-  }
+
+  $prefix   = dmemcache_key_prefix($bin);
   $full_key = urlencode($prefix . $bin . '-' . $key);
 
   // Memcache only supports key lengths up to 250 bytes.  If we have generated
@@ -443,3 +431,42 @@ function dmemcache_key($key = '', $bin = 'cache', $reset = FALSE) {
 
   return $full_key;
 }
+
+function dmemcache_key_prefix($bin = 'cache') {
+  static $prefixes = array();
+
+  // Do we need to load the prefixes?
+  if (empty($prefixes)) {
+    $prefixes['default'] = '';
+
+    // When simpletest is running, emulate the simpletest database prefix here
+    // to avoid the child site setting cache entries in the parent site.
+    if (isset($GLOBALS['drupal_test_info']['test_run_id'])) {
+      $st_prefix = $GLOBALS['drupal_test_info']['test_run_id'];
+    }
+    else {
+      $st_prefix = '';
+    }
+
+    // memcache_key_prefix can be set in settings.php to support site namespaces
+    // in a multisite environment.
+    if ($prefix_var = variable_get('memcache_key_prefix', '')) {
+      if (is_array($prefix_var)) {
+        foreach ($prefix_var as $bin_name => $prefix) {
+          $prefixes[$bin_name] = $prefix .'-'. $st_prefix;
+        }
+      }
+      else {
+        $prefixes['default'] = $prefix_var .'-'. $st_prefix;
+      }
+    }
+  }
+
+  // Try to use a specific prefix
+  if (isset($prefixes[$bin])) {
+    return $prefixes[$bin];
+  }
+
+  return $prefixes['default'];
+}
+