ICS (Android 4.0.3), Samsung Galaxy S2, Exchange Email
I just had the same problem and I found a way of seeing the subfolders on my Galaxy.

On the PC, open your inbox and create a new subfolder (eg foo).

On your phone, open email and open your inbox. in Options, select 'Folders'.
You will see your Inbox with a small green arrow icon. Click the icon (not the Inbox title) and your folder list will expand.

Note that this only worked for me after I had created the extra dummy folder. Before I created the folder, I could see the green icon, but it would not expand the list of subfolders.