Sony has confirmed that Evolution Studios will be release a demo for MotorStorm: Apocalypse, which releases exclusively on the PlayStation 3 shortly.
Currently no date for the demo has been given, but they will be revealed soon enough. As for what the demo will contain, a statement said "If you've seen any glimpses of a track called Mainline, you'll have a good idea of what to expect."
The game has an oddly staggered release, with the UK (18th of March) getting it almost a month before North America (12 of April). From what we've seen so far though, it'll be well worth the investment.