Pysl is a cydia or installer or like the bad boys a FTP install only app but it features looks like it should have been built-in the iPhone OS & not the app store anyway.
So what does Pysl do? Well from the looks of it, the app add features to your default SMS app. It has a built in SMS scheduler, blacklist feature and controls your SMS / text messege notifications.
By the way PYSL stands for protect your secret life.
I have just installed it and going to ty the notifications feature and turn on my SMS Twitter stream and give it a try.
Screenshots of the app below.