Some weeks ago, SiliconValleyWatcher confirmed rumors, first reported by news site GigaOm, that Yahoo! would acquire Flickr, the red-hot photo-sharing service. On Sunday Flickr announced it had sold to Yahoo for an undisclosed sum.

Flickr will almost certainly be integrated into Yahoo 360, the new personal publishing portal that Yahoo will roll out in an invitation-only beta on March 29. Yahoo insiders say that they are still working on how best to combine Flickr with its other media properties.

It's not hard to guess why Flickr chose to be acquired rather than raise a large round of VC money to take the company to the next level, a scenario in which they could easily lose control over the direction of the company. But why Yahoo? Flickr cofounder Caterina Fake explains on the Flickr blog:

The things that were important to us were: being open, building

innovative stuff and kicking ass. Were these people OUR people?

Yes. See the stuff Yahoo's announced recently (including, of

course,this)? They're evolving in really interesting ways --

and from our look inside, we know know that there's a lot more

coming.

Yahoo won't be the Yahoo you've come to take for granted.

Competition (with that other company with two O's in its name)

has done great things for Yahoo. Dude.

That statement speaks volumes to the flipside question, why did Yahoo want Flickr instead of any of the other photosharing sites, many of which probably have better spreadsheets than Flickr? Simply, openness.

In an environment where they are competing against Google and Microsoft, openness is Yahoo's wedge issue. This was a theme that Marc Canter hammered at when I spoke with him at ETech: "Yahoo is No. 3," he said. "They have to go open."

Here's what Yahoo blogger Jeremy Zawodny posted on Sunday:

Combining [Flickr's] mix of tagging, communities, syndication, open APIs, and interactive UI with Yahoo's services and millions of users will lead to even more great stuff. ... [T]his isn't about just throwing millions of users at Flickr or bolting Flickr onto Yahoo! Photos. Think more deeply about it. There are many parts of Yahoo that will be Flickrized in the coming months. And with more resources available, Flickr itself will be able to grow like never before. ... There's obviously more good stuff in the works. For starters, just think about how we could Flickrize Yahoo! 360 (and it's not even out yet!).

By acquiring a photosharing service that owes its fantastic reputation to its open APIs and active developer community, Yahoo is signaling again that they will become the open source portal. By supporting RSS, launching Yahoo Developer Network and getting Flickr, Yahoo is looking like a very interesting company, something that hasn't been heard in the Valley for quite some time.