What is included is a number of software improvements. The bulk of the changes relate to PSN and online play.
First, you'll be able to get notifications for when certain friends sign in. And, perhaps to counter this, you'll be able to appear offline to your PSN friends. For online play, User Scheduled Events will let you set a time and date for a future play session. Once the time is right, anyone signed up for the event will be added to a party, which sounds pretty useful if you regularly game with a particular group. Once you're in a party, you'll be able to see what each member is playing in order to smooth out the process of joining a friend's game. Bringing up the rear is the addition of the popular and totally relevant video app DailyMotion.
There's no clear release date for the new firmware coming to the general public. The last major software revision had month-long beta, which gives us a clue as to the timeline, but how quickly 3.5 will arrive depends on the success and stability of the beta test. Those approved for the beta program should get the update tomorrow, sans Remote Play, of course.