Apple is currently under investigation by the US authorities after Adobe filed a complaint regarding Apple's restrictive iPhone development policies, i.e., only allowing development through its own tools. By also allowing people who use Visual Studio 2010 to write applications for the iPhone and the iPad, Apple basically takes much of the wind right out of Adobe's sails.
There's an added benefit in there for Apple in that people who refused to develop for the iPhone platforms because they don't want to buy a Mac will now be able to develop for the platform with their Windows machine. This will surely allow more people to develop for the iPhone, at little to no cost for Apple - Microsoft is most likely doing the work here.
For Microsoft, it's simply an opportunity to gain a presence in a market where it currently has none. As anyone who is aware of the company's history knows, Microsoft is a very practical company, and isn't religious about any of its choices. It simply goes where the money is, and the iPhone platform is a market. As simple as that.
On top of that, while many people see the world as Apple vs. Microsoft, anyone with a slightly saner mind knows full well that the two companies are actually pretty good friends. These two giants mostly work around each other, and rarely compete directly. Their products mostly compliment each other instead of outright competing. For a recent confirmation, just look at how carefully Apple avoided talking about Windows Mobile when suing HTC - it was all Android.
Of course, this most likely is nothing but a rumour (remember, analysts are bloggers in suits), but the idea itself isn't particularly surprising.