When is an Easter Egg not an Easter Egg?

In a scene in Raiders of the Lost Ark, there are hieroglyphs of the droids from Star Wars hidden amongst the texts.  In Jedi Knight: Dark Forces, Max from LucasArts' 'Sam and Max' is hiding in one settlement.  In Duke Nukem 2, there is a 'doomed' space marine skewered behind an altar.

These are classic Easter Eggs: little hidden nuggets of interest of no consequence.

In Portal, we see the Borealis' dry dock and reference to Black Mesa.

So I ask again, when is an Easter Egg not an Easter Egg?  At what point does a hidden piece of information become 'lore' or 'canon', rather than 'Easter Egg'?  No-one seems to be jumping up and down that Sam and Max must then be in the same universe as Star Wars, or that Star Wars precedes Indiana Jones in some Zelda-esque impossible timeline.

A hell of a lot has been made of both the direct and perceived links between Portal and HL, beyond the realm of fanfic.  But what if they are just Easter Eggs and no intention was ever made to actually follow through on such ideas?

Of course, the Borealis is cited in HL2, but that doesn't guarantee an in-game connection.  If devs followed through on EVERY idea like Aperture being in competition with Black Mesa, story lines would be a right mess.  It could have easily been an afterthought, that once Portal was ready for testing (no pun intended), some bright spark thought, "Hey, Aperture's a science facility, Black Mesa's a science facility, we're all Valve, so why don't we..." and added the slide show content in Portal 1 purely as an Easter Egg, which the 'joke' then continued into Portal 2.

Don't get me wrong, I WANT a HL/Portal cross-over, but one has to temper the vigour sometimes.  Particularly after one of Gabe's recent uncommitted interviews.