According to an interview between the well-established Polish magazine "PSX Extreme" and Sony Computer Entertainment UK, the recently announced switch to Pay-to-Play PSN as far as Multiplayer is concerned covers more than just the future PS4 users. Here's a rough translation of what the "PSX Extreme" crew was told during the interview:
Additionally to this statement, they were also told that the switch also concerns current PlayStation 3 and PSVita users. Understandable, considering the latest figures which show a 95% appreciation rate of the PlayStation Plus service. That being said, it's still a shame to see a previously free service go."Introducing fees for Multiplayer is something that we've debated upon for a long time. The whole matter bothered us for a long time. That being said, I have to admit, thanks to the new form of PS+ we were able to lower the price of the PlayStation 4. It will also allow us to regain financial balance after releasing the new system to the market. Thanks to the new income, Sony is capable of providing the highest quality of Online services which it offers to gamers. One of those features will be the long-awaited Cross-Chat."
EDIT: Due to conflicting information from SCE UK and Mr.Shuhei Yoshida the thread has been marked with the [RUMOUR] tag until Sony provides some further clarification.
Source (Google Translated)