Valvetime.net has discovered several references to what could be a brand new Valve engine buried within Source Filmmaker.
"Return an str with the current engine version. If key doesn't doesn't exist, assume 'Source', otherwise invalid -- assume next-gen 'Source 2'," reads one of the more incriminating examples.
If Valve are indeed working on a next-generation engine, the Episode 3/Half Life 3 delay suddenly becomes a lot more understandable. Since the original Source engine has lasted 8 years already, Valve would almost certainly want to future-proof their new engine as much as possible. This is a rumour for now, but it makes a great deal of sense.