In real life, compasses do not point to geographic north. Instead, they point to magnetic north, and magnetic declination has to be used to correct the result to obtain a bearing for true north.
Currently in ArmA, the declination is +0 no matter where in the world is player (supposedly) is. This is in contrast to the skybox, which is accurate for the terrain's location (at least for Bohemia Interactive official terrains.) Also, magnetic declination lines are already visible on the existing compass model.
Because ArmA 3 will be based on real-world locations, there is an opportunity to implement magnetic declination using the actual declination values. The values could also be adjusted to match the date, just like the current functionality of the skybox. For example, as of March 9th, 2013, the magnetic declination of Lemnos is 4° 21' 37" E and changing by 6.0' E per year.
The magnetic declination value could be stored in the terrain data. To ensure that ArmA 3 remains backwards compatible with existing maps, the declination value can default to +0 (i.e. compass points to geographic north). A magnetic declination calculator can be freely found at http://www.ngdc.noaa.gov/geomag-web/#declination.
If there is consensus that this feature does not fit the "fun vs. sim" balance, it could be implemented as an optional Difficulty setting.