Didn't MS open up a lot of their file formats as part of it's new caring sharing image
Last I heard, that's one of the big issues that they are still holding back on big time. They had a nice little PR campaign where they announced they would allow people to view the actual source code for a fee, claiming to go 'above and beyond' the demands of the European Union's anti-trust lawsuit rulings, but in general they are just ignoring things again, like all along.
File format specifications and API's are a lot more important than being able to view the sourcecode, and they've been fighting the release of that info tooth and nail.
The last couple of office releases have each used slightly different file formats. Originally MS published their specs, but haven't in many years. The support for .doc and .xls and such in competing products at the moment, is all by reverse engineering office documents. They have a very high compatibility level, but still not a 100%.
Another point of how 'open' they want to be, is Microsoft's very vocal disappreoval of the OpenDocument fileformat, which is supposedly intended to be a new universal 'generic' format to enable information exchange between office suits. A bunch of the other vendors have rallied behind it, but Microsoft has pretty much announced they won't support it, regardless of how many requests they get for it.
It's important to remember that Microsoft's monopoly is build on lock-in -- essentially they hold you hostage by locking up your information and making it a major hassle to switch to competing products. Opening up file specifications is one of the biggest dangers in their mind, since it would open up the door for people to leave to a competitor.