The FS100 firmware update that was supposed to land today has been delayed till April 19. From Sony’s FS100 download page:

Firmware upgrades for NEX-FS100U & NEX-FS100UK have been revised for planned release date of April 19, 2012

There seem to be no changes to the features included in the firmware update, namely:

Additional NEX-FS100U & NEX-FS100UK features
1. Added 50Hz recording system (PAL): 1920×1080 50p, 50i, 25p and 1280×720 50p.
2. Camera Profile (camera settings can be saved to and loaded from a memory card).
3. Additional 4x and 8x Expanded Focus magnification: (with user selectable focus area).
4. Selectable ISO or GAIN sensitivity display.
5. Selectable Focus indication in FEET or METERS (E-mount lens only).
6. Selectable Shutter indication as EXPOSURE TIME or SHUTTER ANGLE.
7. Added the following Aspect-Ratio markers: 2.35:1, 1.85:1 and 1.66:1.
8. Display ON/OFF button enables Zebra and/or Histogram overlay onto video output.
9. Compatible with the new A-Mount to E-Mount adaptor, model LA-EA2. One push Auto-IRIS and continuous 15-point, Phase-Detection Auto-Focus operation with most Alpha A-Mount lenses is available. Lens metadata is displayed onscreen and stored real-time to image file.

Until we get that new ISO display option, you can continue using the Gain dB to ISO equivalent table that Abel Cine first posted last year. Note that they updated their FS100 dynamic range test in this later post.

Gain db Level ISO Rating 

0 db 500 ISO
+3 db 800 ISO
+6 db 1000 ISO
+9 db 1600 ISO
+12 db 2000 ISO
+15 db 3200 ISO
+18 db 4000 ISO
+21 db 6400 ISO
+24 db 8000 ISO
+27 db 12800 ISO
+30 db 16000 ISO