In a long interview on the Playstation Blog, Sony have made it clear that PS Plus will be key to the PS3 experience.
Sony stated that PS Plus is a “core part of the way forward” and hence a core part of the PS4 proposition . Interestingly whilst the Sony team also highlighted how strong the existing (and unannounced) line up for the PS3 is they also see that the transition from PS3 to PS4 will be faster than that from PS2-PS3.
This is probably due to the PS4 using less propriety hardware, which will allow costs to fall sharply. Hence there will be less reason to keep the PS3 in the market place for as long as the PS2 has stayed in play.
In some markets and for some consumers (Latin America, the Middles East etc) the PS3 is still very expensive and hence the PS2 has allowed Sony to have a two tier approach. The PS4, potentially, will allow them to move to a one tier offering within a couple of years of launch.