Microsoft is expected on Wednesday to announce a partnership with European mobile giant Nokia to help get its Office software onto that company’s mobile phones.

With the next version of Office, Microsoft is trying to expand its desktop hold on the productivity market into one that spans the PC, Web, and phone, and this deal is seen as a significant move in that last category.

The software maker has already said that, with the next version of Office, it plans to offer browser-based versions of Word, PowerPoint, Excel, and OneNote. Those programs will be able to run inside Safari and Firefox in addition to Microsoft’s Internet Explorer. That means that Office, for the first time, will run on Linux-based machines.

On the phone side, Microsoft has shown the ability for Office 2010 documents to be displayed on a variety of mobile phones. So far, the only phones that have their own native versions of Office have been those running Microsoft’s Windows Mobile software.

Microsoft is looking for ways to strengthen its Office franchise into one that maintains its relevance and market share even as the PC becomes just one of many devices people use to access their information. Office is also vital to Microsoft’s fiscal health, with much of the company’s profits still coming from Windows and Office.

Microsoft released a technology preview of the PC-based Office 2010 applications in July, although it has yet to start publicly testing the browser-based versions. The final version of Office 2010 is due next year.

Although Nokia and Microsoft have long been rivals in the phone business, the two have also struck deals at times. Nokia already has a license that allows its phones to connect to Exchange Servers using Microsoft’s ActiveSync protocol. In 2007, Microsoft also struck a deal with Nokia to have Windows Live services run on the Finnish company’s phones.

The deal comes even as Microsoft is trying to figure out how to keep its Windows Mobile operating system in the game amid stiff competition from Nokia in Europe as well as Apple’s iPhone, RIM’s BlackBerry, and an emerging threat from devices running Google’s Android operating system.

Expanding Office to other mobile devices may help that business, but at the same time takes away one of the areas where Windows Mobile had a leg up on rivals–its direct compatibility with Office.

Update: The two sides aren’t talking details, but they have confirmed a press conference on Wednesday to discuss an alliance. It will start at 8 a.m. PT and CNET will have live coverage.

Also, as TechFlash’s Todd Bishop points out, Microsoft’s Mac Business Unit has scheduled an announcement for Thursday. Do you think we’ll see Office for the iPhone this week?