I read via ShackNews this morning that the guys over at ModHQ have found out that the HL2: Aftermath expansion pack will be released on April 24th, which coincides with the release of the platinum edition of HL2.

"After reading an internet report which claimed HL2: Aftermath would be released on the 24th of April while HL2: Platinum would be available much earlier, on the 28th of January, I decided to e-mail Valve Software's Doug Lombardi in order to ask for a confirmation. This is the response I received (many thanks to Doug):"