SDS stand for Sudden Death Syndrome and this is a common term use nowadays to indicate a phone die without noticed.
For Samsung S3 it was very common to early batch from May 2012 to late Jun 2012, in short those who brought during launch date. So far no concrete root cause and many speculate following:-
1. Only effecting early batch from May 2012 to late Jun 2012 but many early batch still running strong.
2. Many said due to insane Chip in early set but this is only one developer speculation base on information gathered.
3. Many said due to overnight charging but smart phone should cut off when charge is full.
4. Many believe latest 4.1.2 will fix the problem but some believe harm already started and FW upgrade will only slow them down.
5. Many said latest MOBO eliminate the problem but someone who change MOBO to latest version M.P. 1.5 die twice.
6. Many scared and sold their phone in early stage.
7. Many hope to get SDS before warranty expired, like me

but no luck.
8. Many said due to Custom ROM but Stock user also facing the same problem.
this.....is very disturbing information....
but thanks for the info.
i shall wake everyday in fear that my phone cant be turned on.