Submodule commits not show by git log

Hi all,

I'm trying to track changes on a git submodule. `$ git log
path/to/submodule` is not showing any commits that changed the
submodule commit.

For example, in my repo I have a submodule at the path
./memcache_utils that has had the commit referenced changed several
times. It's definitely a submodule:

$ git submodule

13eb087304b995705693d6f0927dec2d88dfadaf datastore_utils
(heads/master-2-g13eb087)
 7f5d6710b767a27f14e3e7bc009f026b3e5f0e74 memcache_utils (heads/master)
 5877e2c2d82645fa44f121884291ee48cf24584d potatobase (5877e2c)

Yet the only commit shown is when there were files at that path:

$ git log memcache_utils
commit 2cbe65bf31901873b01331e95fec72724e7458eb
Author: [...]
Date:   Thu Jan 26 20:44:07 2012 +0000

    Experimenting with Paul G's port of cache-machine. It should
really be a git submodule but just want to see how it works so have
taken a copy from [...]
$

Have I missed something? I've tried and failed to create a minimal
test case that demonstrates this behaviour. I can't see anything in
the man pages to suggest that I'm using git log incorrectly, and
Googling doesn't help.

Is this user error, a known bug, or a new one?

Many thanks
Wilfred