Using 1.4.7 on OS X 10.5.1, re the GnuPG prefs, whatever text is entered in the comment field (as noted above), is lost when navigating back to the tab.
As a result, the comment field is blank.
May be related to bug #1345534 (which doesn't appear to have been addressed since first reported in 2005).
Logged In: NO
Further testing shows that the comment can be successfully input using Expert Mode - when this is done, the text is reflected in the comments field in the signature tab.