[magnusandersson]

Problem solved!

I emailed the support and got this answer:
"The week number that OmniPlan uses is pulled from Mac OS X's settings and preferences. If you go to System Preferences > Language & Text > Formats and set the region to Sweden or United Kingdom the week number should reflect the numbering used in Europe. Make sure to quit and relaunch OmniPlan after you change this setting in your System Preferences."

Thanks Omni Group!
/Magnus