All this is a grain of salt to be sure, BUT:
I have thought for some time (even before this whole portal 2 show/no show thing) was that the delay for HL2-EP3/HL3 was due to Valve updating the Source engine. Wether this means "Source 2" or not, I dont know. But what I THINK is that it is some kind of new Source..and that it is MULTIPLATFORM.
Without starting a piracy debate the fact is ALL major game engine developers are now in full swing of working on multiplatform support. It is the only way to remain profitable and viable in todays enviornment. The reasons for this are varied, but PC piracy is a big one (I know steam eliviates this SOMEWHAT, but expanding to support consoles is a logical step, and "everyone is doing it")
Second. It "gets them off the hook" with regard to HL2-EP3/HL3 delays. Showing off a new Source (and PERHAPS HL"3" running on it) would go a long way to appease (or at least "bait-and-switch") the complainers about the half-life delays. And gives them more time to now work on it without the naysayers reaching fever pitch, or at least, the inevitable comparisons to Duke Nukem Forever. ;-)
Third. That VG247 article did say something about the announcement coming AFTER Microsoft and Sony's keynotes. This may play into the hands of an announcement of Source becoming multi-platform.
That being said, apparently, Gabe Newell will NOT be at E3. I, for one, would think that if there really was an announcement like that, he would be the one to do it.
Fun to speculate,but I guess we will all know in a bit over a week...