After scouting through the interwebz for a more scientific explanation, I discovered that Vaughan Bell had written up a good explanation on his excellent Mind Hacks blog. I hope he won't mind me nicking it, but I think it's that good that it deserves repetition:
According to press reports Michael Jackson will be buried without his brain because it is still 'hardening'. Although this may seem unusual, the 'hardening' process is actually a standard part of any post-mortem examination where the brain is thought to be important in the cause of death, such as in suspected overdose.
It involves removing the brain from the skull and leaving it to soak in a diluted mixture of formaldehyde and water called formalin. This soaking process usually takes four weeks and the brain genuinely does harden.
A 'fresh' brain is a pinkish colour and has the consistency of jelly, gello or soft tofu meaning it is difficult to examine and the various internal structures are often hard to make out.
After soaking the brain, it has the consistency and colour of canned mushrooms making it easier to slice, examine and photograph. However, because the brain is so soft to start with, it can't just be dropped in a tank of fixing solution, because it will deform under its own weight.
To solve the problem it is usually suspended upside down in a large bucket of formalin by a piece of string which is tied to the basilar artery.
After it has 'hardened' or 'fixed' it is sliced to look for clear damage to either the tissue or the arteries. Small sections can also be kept to examine under the microscope.
Because this part of the post-mortem takes several weeks preparation it is usually only carried out with the family's permission as the body may need to be buried without it, or the burial delayed until the procedure is finished.
This also means that this form of post-mortem brain examination is usually only carried out where there is a feeling that examining the brain can help clarify the cause of death - which is what pathologists are often most concerned with.
In cases such as Michael Jackson's, where the effects of drugs are suspected to play a part, pathologists will be looking for evidence of both sudden-onset and long-term brain damage. If they find it, they'll be trying to work out how much it could have been caused by drug use and how much it contributed to the death.
So now you know.