Microsoft's been touting its Live Tiles concept ever since Windows Phone 7 launched two years ago, but the launch of Windows 8, Windows Phone 8, and particularly the Surface tablets appear to have given a company called SurfCast an opening to file a patent lawsuit over the concept. Microsoft's held high-profile launches of all three products lately, so news of the case is being reported widely — but a basic examination of SurfCast's patent reveals Microsoft's move into selling its own PCs is the key to the complaint.
SurfCast owns US Patent #6,724,403, which was filed in October 2000 and issued in April 2004. Broadly, the patent covers selecting a variety of information sources, assigning each of those sources to a tile, and updating those tiles at variable refresh rates. That's very much what Microsoft's Live Tiles do, but there's a catch: SurfCast's patent claims are for "a device under control of a program" and "an electronic readable memory to direct an electronic device," not pure software.
Most "software patents" are really hybrids of hardware and software
Most of what we think of as "software patents" are really this type of hybrid hardware / software claim — pure software patents were disfavored until fairly recently, so lawyers and the USPTO would pretend that software turned a general-purpose computer into a specialized machine, and then try to patent that machine. (This sounds ridiculous, and it is.) Until the Surface, Microsoft was in the business of selling pure software, so it wasn't really an attractive target for a patent lawsuit based on these types of hardware / software claims. It's sort of the same situation as Apple and Google — Google doesn't make any hardware, so Apple's been focused on taking Android hardware manufacturers to court.
To be clear, SurfCast's complaint also accuses Microsoft of inducing its partners to infringe the '403 patent and contributing to that infringement, so it could have filed this lawsuit earlier based on Windows Phone 7, but being able to accuse Redmond of direct infringement obviously makes for a much stronger case. (And, well, Microsoft's sales of Windows Phone 7 don't exactly add up to a ton of money.) The timing could also certainly have been influenced by failed negotiations between the two companies — SurfCast has been around since 2001 and owns three other patents, but doesn't appear to make or sell anything of its own, so it's undoubtedly been trying to license its portfolio in true patent troll style for some time. It's probably a safe assumption that Microsoft will reach some sort of settlement; these types of cases rarely linger on.