I disagree pretty strongly with LP's accidental chloroform/taped mouth to quiet screaming theory.
Reports about the duct tape have varied-- some say it was allegedly found over the mouth area. Other reports have said "wrapped around the skull."
If the tape was simply placed over the mouth area, it was IMO used to stage the body so that it looked like there was a kidnapping. This staging was informed by the Hollywood/TV template for kidnapping.
If you're not going anywhere tonight, I invite you to put a strip of duct tape over your mouth. You can still make enough noise to draw attention, and you can get the tape off rather quickly, using your mouth and facial muscles alone.
To effectively secure tape over a victim's mouth, the tape needs to be wrapped around the skull. If you have hair on your head, do not attempt to cover your mouth with duct tape and secure the tape by wrapping it around your head.
There's no way Casey was routinely drugging Caylee and effectively using duct tape to quiet Caylee (by wrapping it around her head)-- Caylee's hair would have been very damaged and broken and I'm sure Cindy would have noticed.
So, IMO, either a strip of tape was placed over Caylee's mouth to create the appearance of a Hollywood-style kidnapping, or the tape was wrapped around her head during the course of, or after, a very calculated murder (either to quiet Caylee during the murder or to more realistically stage the scene).
If the tape was wrapped around the skull, the killer had no intention of ever removing the tape from a living Caylee's head, IMO. Also IMO, the tape was left there for a reason-- to give the appearance of a kidnapping.