What is SAB?
Paul - while zinedi describes the symptoms of SAB, he fails to mention this was an issue for a small number of 2011 build cameras. This being a camera forum, there are many reports of SAB here as you'd expect, but don't be misled that it will "inevitably" happen.
You can tell the manufacture date 2 ways. The easiest is the serial number which starts with 2 numbers and a letter, the first digit denoting the year where 1=2011, 2=2012 etc and the second digit is the quarter within the year, 1=Q1, 2=Q2 etc. The letter is the localisation (A=USA, N=UK are the only ones I know). So serial number 21Nxxxxxxxxx is a Q1 2012 UK retail camera, and would not be affected by SAB. I believe Fuji begins serial numbers for all its cameras the same way.
For a more precise manufacture date (i.e. the exact date), the X100 actually embeds this in the EXIF of every photo, though many photo applications do not read it. You can get an EXIF viewer application/web browser plug-in that will show this.