I see little advantage for either company with Microsoft outright acquiring Adobe. Both companies have seemingly exhausted their growth opportunities in their primary markets. I don't foresee too many more people switching to Photoshop from a competitive product, or people switching from linux to Windows. Growth in these markets is out of the question. But, there are plenty of places to setup tollbooths, to make-up for lost potential revenue.
Both Adobe and Microsoft would love to have a bigger slice of the mobile market. With Apple snubbing Adobe at every opportunity, and Microsoft getting caught with their pants down with the release of the iPhone, there is no shortage of men holding grudges in Redmond or San Jose.
If you look at the most popular games on the iOS devices, many have roots in Flash based games. In the short run, if Adobe released Flash or something Flashy, for Windows Phone 7 to get access to Flash game developers, this could only help with Windows Phone 7 and mobile Flash uptake.
Windows printing and document management is clunky at best. Adobe has all kinds of issues with security, and Microsoft as been working really intensely on security for a while now. If Adobe could integrate a better print workflow for Windows, and Microsoft could help with hardening Adobes applications, everyone wins.
The Expression suite of products has a good head start with importing documents from Adobe apps. However, the relationship isn't nearly as symbiotic as it needs to be. There is a lot of potential for XAML in the future, it performs it's duties well, separating code from markup, while maintaining expressiveness for both programmers and designers. Adobe has a suite of killer design tools, that have desperately needed a feature set extension worthy of a version bump. I think you could get that set, with better integration with the world of developers. Microsoft at it's core is about developers, as Adobe has designer DNA.
What the future needs is a chimera, how we are going to get there, I don't know. But I don't think we will get there with a merger of two adult companies. I would like to think that something can be created with a partnership; something like what GM and Toyota had in Nummi, with the resulting offspring being a big win for consumers, developers and designers.