I just got a call from SpaceX communications director Christina Ra with a heads up: testing at their McGregor development facility is about to get loud even by their standards. (Given recent events, it was considered an especially good idea to let people know about this ahead of time.)
In keeping with the company philosophy that tests happen as soon as everything's ready rather than holding to a rigid schedule, Ra couldn't say yet precisely when the particularly loud tests will happen. She said a short, 10-second test could come as early as Tuesday, April 30; a test firing for the full 3 minutes a Falcon 9 rocket's first stage burns on the way to orbit would follow a few days after the first test.
Ra couldn't comment on precisely what technology was being tested — SpaceX tends to keep as mum as possible on such details both for proprietary reasons and out of worries about violating federal law against aiding foreign weapons development — so this is where the speculation begins.
The most likely possibility is that it's a Falcon 9 first stage test using nine of the new, more powerful Merlin 1D engines (it's known that the 1Ds have been tested individually; this could be the first test of the full, upgraded Falcon 9 v1.1 first stage). The more outside shot is that this is the start of testing on the 27-engine Falcon Heavy first stages, but since the Heavy is also set to use Merlin 1Ds it seems likely that they would want to test the nine-engine variant first.
I'll hopefully know more over the weekend. Stay tuned.
UPDATE, 4/30: This almost certainly falls under the heading "Barn Door Closed Too Late," but those of you linking here from CSExtra and elsewhere over the weekend: Hello, and here's the followup.