It is possible to decode the WiiFit save data.
Once the WiiFit savedata is stored to a SD card it will be named private\wii\title\RFNP\data.bin (for pal) or private\wii\title\RFNN\data.bin (for NTSC)
This is a standardized Wii format that all games use and it is described at http://wiibrew.org/wiki/Savegame_Files
Once you have decrypted the header and data area with he keys from http://hackmii.com/2008/04/keys-keys-keys/ you will find that data.bin contain the files:
RPFitCap.dat
RPHealth.dat
RPWiiFit.dat
These files are unencrypted, but I have not analyzed their content more then just to be able to extract weight and bmi data for my own Mii.
I have a really dirty vb6 class that produces a CSV file with dates and weight but its faaaaaar away from any kind of release.
Heres some of my extracted data:
15.11.2008 13:18:00;92
16.11.2008 15:30:00;91,1
17.11.2008 19:02:00;91,3
18.11.2008 08:23:00;90,8
19.11.2008 07:20:00;90,5
20.11.2008 09:34:00;90,5
21.11.2008 09:32:00;91,1
22.11.2008 09:11:00;91,3
23.11.2008 10:25:00;91,6
24.11.2008 10:36:00;91,2
25.11.2008 10:37:00;91,4
26.11.2008 13:40:00;90,8
27.11.2008 10:45:00;91,2
28.11.2008 11:32:00;91,4
29.11.2008 13:09:00;91
30.11.2008 13:18:00;90
01.12.2008 12:38:00;90,1
02.12.2008 13:16:00;91,2
03.12.2008 10:34:00;91,2
04.12.2008 12:06:00;91
05.12.2008 13:05:00;91,2
06.12.2008 16:28:00;90,3
07.12.2008 14:03:00;90,9
08.12.2008 12:38:00;91,3
09.12.2008 14:18:00;90,4
10.12.2008 13:43:00;90,5
11.12.2008 13:36:00;90,5
12.12.2008 14:15:00;90,3
13.12.2008 14:17:00;89,9
14.12.2008 10:42:00;90
./Al