Microsoft will never ever open their source, nor are they likely to pay the full $615m fine. They might submit to the unbundling of WMP. For those who can't understand why, let me explain.
First, the source. We all know that MS will fight against this more than any other single punishment. This is what they truly don't want to have happen. You can bet that MS would be willing to go to such extreme's as pulling out of the EU market rather than supply the source. Further, the EU has no real way to actually force them to release it anyway. They can issue such an order, but if MS says no, what are they going to do? They can't legally seize the code and release it for them (at least, I don't think they can). The only way to force MS to release the code would be through the cooperation of the US, which isn't likely to happen. Regardless of whether or not the US gov agree's, as soon as the EU tries to force their hand, it becomes an EU vs US thing (guess who will win that battle).
Second, the fine. It's big. Damn big. Yes MS can technically afford it, but if nothing else, that is likely to get overturned and reduced to $100m or so.
Finally, the unbundling. If MS releases an unbundled version it will cost exactly the same as the regular version, because hey, WMP is free, right? Second, people who mistakenly buy the unbundled version are gonna be peeved when they can't find WMP and are gonna complain. Whether the Open Source/Free Software communities likes it or not, people want this software, like this software, have gotten used to this software, and most importantly, DO NOT CARE THAT THERE ARE ALTERNATIVES. Yes, thats right, they are happy using WMP and want to keep on using it. So what is ultimately going to happen? People will buy the bundled version anyway! Furthermore, MS knows this and is more likely to comply for just this reason. They've already strangled the market, so it doesn't matter anymore.