I think that was built into the sense roms. It's not very hard to put the phone on vibrate though, pull down notification and tap the bell icon where it says "normal" and it should set it to vibrate.
Sent from my MIUI Incredible.
I believe it slightly underclocks the processor when the screen is off and to keep from allowing "rogue" apps to consume a lot of power.
Sent from my MIUI Incredible.