The Finnish company is now the only major mobile manufacturer not to offer Microsoft's Windows Mobile OS as an option - aside from Apple, of course. So is Sony Ericsson really harpooning its trusty Symbian software? No, of course not. But it does mean that Sony Ericsson now sees Windows Mobile as an OS that's finally going places.
Windows Mobile: a slow starter
Sooner or later, Windows Mobile will have the features that will make it stand out from the pack. At the moment, it runs like a dog with three legs, but it's pretty clear that this will change. As any Microsoft watcher will realise, Redmond needs several bites of the cherry before it gets something right. Or as near to 'right' as it can.
Crucially, Microsoft can't waste any more time, not with the immediate threats to its operating system posed by Apple's iPhone and Google's forthcoming Android platform. Microsoft also knows that natural, intuitive interfaces will be a big focus over the coming years and it seems determined to meet that challenge head-on.
John Starkweather, director of international marketing for Windows Mobile, told us at Mobile World Congress that the Sony Ericsson deal is part of Microsoft "broadening the appeal [of Windows Live] to the consumer."
Will Nokia kiss up to Microsoft?
So what's going on with Nokia? Nokia's president and CEO Olli-Pekka Kallasvuo gave a clear "no current plans" when asked whether the company had a Windows Mobile-powered phone in the pipeline. That's not to say it won't do one in the future. But it certainly wouldn't seem to be in the company's current thinking.
Despite popular opinion, Microsoft and Nokia seem to like each other. They're already in partnership in several areas including Microsoft's PlayReady and Windows Live. Sony Ericsson won't be turning its back on Symbian - it part owns the company and the OS has proved its worth in the lower-end CyberShot and Walkman phones.
But if Apple's iPhone is any indication of the smartphone trends to come, it's no wonder that Sony Ericsson wants to have an egg in the Windows Mobile basket. Microsoft is becoming increasingly aggressive in the mobile space - its recent acquisition of Danger displays a desire to become more consumer orientated and drive mass adoption. And this is only reinforced by the sheer size of its presence at this year's Mobile World Congress.
The new Sony Ericsson Xperia X1 phone is a taste of ambitious handsets to come.
The Symbian empire will strike back
Nokia, meanwhile, is forging ahead with its plans for Series 60. Sony Ericsson isn't using Nokia's S60 iteration of its Symbian-based OS, but others are. The LG KT-610 and the Samsung G810 are two new launches that use S60, which is expected to support touch input this year.
In a briefing at Mobile World Congress, Nokia's vice president of Software Sales and Marketing Matti Vänskä said 150 million S60 devices had now been shipped worldwide, while the company was focused on bringing "new, innovative and intuitive ways to interact with device".
Since Nokia is the key driver behind Series 60 it will want to do as much as it can with its OS. But ignoring other operating systems could be a mistake - especially if Microsoft can iron out the creases in Windows Mobile and make it appeal to the average Joe.
Get daily insight, inspiration and deals in your inbox
Get the hottest deals available in your inbox plus news, reviews, opinion, analysis and more from the TechRadar team.
Dan (Twitter, Google+) is TechRadar's Former Deputy Editor and is now in charge at our sister site T3.com. Covering all things computing, internet and mobile he's a seasoned regular at major tech shows such as CES, IFA and Mobile World Congress. Dan has also been a tech expert for many outlets including BBC Radio 4, 5Live and the World Service, The Sun and ITV News.