There was an autopsy performed on Caylee even if it was just bones. Cause of death was undetermined because of the amount of time she was exposed to environmental conditions and all that was left were her tiny bones
. MOD was homicide per Medical Examiner (Dr. G).
My understanding (could well be incorrect) was that a traditional autopsy was impossible (as only bones remained) and the examination of the skeleton was largely completed by the Anthropologist Dr. John Shultz-- with the entire operation supervised by the medical examiner.
An autopsy report was released of course, and a cause of death given (Homocide by indeterminate means) given.
In Celina's case, unfortunately cause of death was undetermined as well and her death is ruled as suspicious which like you said could mean anything from murder, accident or suicide. Wish we had more answers.
Yes, from my reading it means "We don't know."
However, and this will be heartening for those hoping for justice in this case, undertermined can easily change. The ruling is a conclusion reached not just from an examination of the physical clues the body reveals, but also from the evidence surrounding the victim. To elaborate, based upon my reading:
In this case we have (based upon what we know) a dead girl discovered underwater. It is not always possible to determine if she was or was not dead when she entered the water, nor is it a simple matter in cases like this to determine if injuries to the body occured before or after death. Nor, apparently, does the manner in which the body was discovered lead to a "beyond a reasonable doubt" conclusion about the cause of death on the part of the medical examiner. Lacking a mechanism of death and a reasonable conclusion as to cause, a ruling of undetermined is appropriate.
For example, in the case of Caylee Anthony, the girl was reported missing 30 days after she apparently went missing. Her skeleton was found in a swamp months later, with evidence of duct tape covering her face. The Doctor's involved in examining her were unable to determine the exact mechanism of death with absolute certainty; but the duct tape, failure to report, and disposal of the body led them to rule that the cause of death was a homocide based upon this reasonable doubt standard.
In other cases (see the links I posted above) it is apparently all too common in cases of suicide for the medical examiner to issue a finding of undertermined, based on nothing more than uncertainty about the situation surounding the death and sensitivity to the family. If the question is "Why is this person dead" then, in cases of apparent suicide, the answer they seem to arrive at a disproportionate amount of the time, is "Welll... we are not really sure."
Which is NOT to say that I believe Celina's death was a suicide. In Celina's case it is important to keep in mind that they may have a very good idea as to the mechanism of death but lack only some additional information, as additional evidence becomes available it is possible that this ruling could change. After reading up on this further I believe I now understand why the Medical Examiners office felt a toxicology report was so important.
NOTE: I AM NOT AN EXPERT OR EVEN KNOWLEGABLE IN THIS FIELD!!!! 
