This patch prevents the offending self-block in the first place: " !cat/pkg " is not replaced.
--- /usr/lib/portage/pym/portage/update.py	2012-10-13 18:39:58.103791174 +0100
+++ update.py	2012-11-07 22:04:23.202408168 +0100
@@ -45,7 +45,10 @@
 		# EAPI (characters allowed in package names may vary).
 		if old_value in mycontent and isvalidatom(new_value, eapi=eapi):
 			old_value = re.escape(old_value);
-			mycontent = re.sub(old_value+"(:|$|\\s)", new_value+"\\1", mycontent)
+			# move slotted
+			mycontent = re.sub(old_value + ":", new_value + ":", mycontent)
+			# move plain category/package (keep self-blocks in mind #367215)
+			mycontent = re.sub("(^|[^!])" + old_value + "($|\\s)", "\\1" + new_value + "\\2", mycontent)
 			def myreplace(matchobj):
 				# Strip slot and * operator if necessary
 				# so that ververify works.
@@ -55,7 +58,9 @@
 					return "%s-%s" % (new_value, matchobj.group(2))
 				else:
 					return "".join(matchobj.groups())
+			# move with version specification
 			mycontent = re.sub("(%s-)(\\S*)" % old_value, myreplace, mycontent)
+
 	elif update_cmd[0] == "slotmove" and update_cmd[1].operator is None:
 		pkg, origslot, newslot = update_cmd[1:]
 		old_value = "%s:%s" % (pkg, origslot)