The glibness spell, for one example. It gives a "+20 bonus on Bluff checks made to convince another of the truth of your words" -- notice that no type is given. That makes it an untyped bonus. However, you cannot cast glibness twice to get a +40 bonus. The bonuses are both from the same source (the glibness spell) and thus do not stack. If there were another spell, let's call it gift of gab, that gave a "+20 bonus on Bluff checks to mislead another person", it would stack with glibness and give a +40 total bonus, even though they both essentially give a bonus in the same situation. The bonuses are still from different sources.
What if there was a second spell, of different level that gave an untyped bonus, would you allow both spells to stack with each other?
I know we are now charting into the extreme "maybe" and "what-if" but I am not completely convinced. As a GM I know that ultimately I have to decide which way to make the ruling and I don't want to dampen a player's fun but I also want to have a reasonable challenge. Heroic is heroic but once he hits lvl 10, that +5 bonus becomes a +10, which given all the possibilities he could under the current rules have the following Diplomacy:
+3 Class Skill
+8 Stat Bonus (20 base with +6 CHA item)
+6 Skill Focus
+5 competence bonus from item
Total +36 Diplomacy
I am almost positive that in 3.5 those two feats didn't stack ... I'm curious as to why the change.