As I see it, nibbana as in the cessation of the aggregates, includes the cessation of feeling. So "where there is nothing felt" means where there is no feeling. And since consciousness can't exist without feeling (MN43), the pleasure he speaks of is not a sensation, but the absence of sensations.Modus.Ponens wrote: If pleasant sensations are not happiness, why would the pleasure Ven. Sariputta speaks of, be happiness?
If we look at SN 38.14 we see three types of dukkha. I think we can say that normally we speak about happiness if we can be free of the dukkha of pain. However, there is still the more subtle types of dukkha of fabricating and impermanence which are intertwined in the aggregates, intertwined in feeling.
But if we want to understand this and realize this, I think reading suttas and intellectual pursuit is not the best. If we follow Sariputta's advice, it's best to investigate the process of cessation in the jhanas.Be it a pleasant feeling, be it a painful feeling, be it neutral,
one's own or others', feelings of all kinds —
he knows them all as ill, deceitful, evanescent.
Seeing how they impinge again, again, and disappear,
he wins detachment from the feelings, passion-free.
http://www.accesstoinsight.org/tipitaka ... .nypo.html