The pack isn't included in the game's Season Pass, but your Porsche Expansion Pack won't just get cars, either: there will be 20 new events, 10 XBox Live achievements and a Rivals Mode all based on Porsches as well. The pack is scheduled to be available on May 22 and will take FM4's car count (with DLCs) to 625. The licensing deal isn't long-term, so if Porsches are your thing you should mark May 22 on your calendar.
The other two new events on the Forza 4 horizon are that level caps have been raised from 150 to 999, and a new game called Forza Horizon is on the way. It's being developed by an outside studio called Playground Games and no one seems to know anything about it beyond Turn 10's assessment that "it's a fast-paced pick-up-and-play racing game." There's a video trailer for Horizon after the jump that won't tell you anything more than you already know, but sounds pretty cool.