Whilst I do like the idea of this, I have some fears.
First, this is a very innovative approach, letting modders officially have access to the source code is something I've personally never heard of (but it probably has happened). It is an excellent plan to improve upon already great games and perhaps breath new life into them. If the modders must sign a NDA in order to access the source code that is alright as well, although I still don't know how they are going to decide who gets to view it and who not. An NDA is a powerful thing but if this was opened to many people, then I can't help to think that the source code may be leaked, and it only takes one person for ireperable damage to be done. A tighter leash can be put on the modders by limiting the number of people involved but then people may be left out.
Although I am very bad at C++ I still have a basic knowledge of it. If I were to request the source code, would I be allowed to view it, knowing that I can almost certainly not develop a mod for any game? If I wasn't allowed to see it, then I would be sad, but if I did then many people can have access to it, creating less security.
The thing I don't like most of all though is the mixing of money/profits with this. I more than understand that paradox has to make money. It is a company, it exists principally to make money. But I don't like paying modders. It dosen't feel right. IMHO you can't have it both ways. You cannot ask the community to make things for you for profit. It would create an uneasy dynamic between developer and modder. Paradox would still make money off free releases, I predict. Having improvements to older games would rejuvinate their sales and if they had to be compatible with the expansion packs to the various games, then people would buy them even more.
I think that if paradox were to release its source code, be it to a select few or to anyone who wants, their output must be downloadable for free. So, you run the risk of individuals getting the source code and compiling the games themselves or whatever but most people can't do that and it would probably be easier to get a torrent or something. I'm sorry for being cynical, but I just can't imagine this going as Paradox intends unless they loosen everything or tighten everything. Maybe Sweden is too progressive even for me. I hope it works, I really do, thats why I voted in favour, but I hope more than that that this doesn't distance me from paradox even more than EU3 did or as I suspect EU:R will.