Module docstring indicated seek() isn't implemented, but it is.
diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index 5d9597f..09713e3 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -79,7 +79,7 @@
" an_input_stream=StringIO(a_string)\n"
" spam=an_input_stream.readline()\n"
" spam=an_input_stream.read(5)\n"
-" an_input_stream.reset() # OK, start over, note no seek yet\n"
+" an_input_stream.reset() # OK, start over\n"
" spam=an_input_stream.read() # and read it all\n"
" \n"
"If someone else wants to provide a more complete implementation,\n"