The mail archive function (UI introduced through bug 482458) creates archive folders as needed. With default settings, one folder per year is created below the Archive folder. If the Archive folder is set to be created on an IMAP server (in my case Courier), only the Archive folder itself is shown in the folder pane directly after the first archiving and there is no expander (+) in front of it. Looking at the subscriptions one can see that both the Archive folder and the <year of archived message> subfolder have been subscribed, though.
Workaround: Closing and re-expanding the corresponding account is enough to fix this situation.
1. start SM with patch from bug 482458 applied (should be on trunk soon)
2. press Shift+A (Archive) on any message where the corresponding server's Archive folder is set to an IMAP server (for IMAP accounts this is the account itself by default)
3. look at the folder pane
Actual results: only the Archive folder is shown, with no expander in front of it
Expected results: the Archive folder should have an expander which, when clicked, exposes the newly created subfolder <year of archived message>
Fixed through bug 573392 (EnsureFolderIndex call):