Swedish newspapers are full of reports today. It is said that Jonas' affair with the Norwegian girl took place in April 2009, that Madeleine found out but still accepted his proposal, because they were determined to stay together.
Apparently Swedish newspaper Aftonbladet has received lots and lots of tips about Jonas' partying from people during the last few months, but they have chosen not to publish out of respect for Madeleine.
Jenny Alexandersson: Ska de satsa på sin relation eller gå isär? | Jenny Alexandersson | Victoria gifter sig | Nyheter | Aftonbladet
I actually believe them, since our newspapers are fairly humane compared to the British ones for example. We have not had any big scandals in our royal family, so the media are not used to handling rumours about royals or nearly-royals cheating.
Of course, it is impossible to know anything for sure.