Wikimedia Bugzilla is closed!

The monobook stylesheet has the definition:

/* External links */
#bodyContent a.external {
	color: #36b;
}

This definition overrides the 'less specific' rules:
.mw-body a.external {
	color: #36b;
}
.mw-body a.external:visited {
	color: #636; /* bug 3112 */
}
.mw-body a.external:active {
	color: #b63;
}

Resulting in the monobook skin no longer having a distinct color for visited links. We probably have this issue with more elements and/or more skins.