For Mac users, Microsoft (MSFT) Office, like death and taxes, is something you can’t escape.

With half a billion users worldwide, programs like Word, Excel and Powerpoint are as ubiquitious as mosquitoes and as likely to draw blood. Apple (AAPL) makes its own applications suite, iWork, with programs called Pages, Numbers and Keynote that fill the same ecological niches. But to communicate with the outside world, you need to know about Office. That’s why three quarters of the Macs in use today run some version of it.

The current edition, Office for Mac 2008, is big (1 GB), expensive ($399.95 for business, $149.95 for students and home) and much maligned for introducing new file formats (.docx, .xlsx) that caused endless compatibility headaches.

Compatibility was one of design goals of the next iteration, which will skip four years and be called Office for Mac 2011 when it ships next fall. We got a demo — well, more like a peek at some static screen shots — at Macworld. Here are the highlights:

• Collaboration. There’s heavy emphasis on co-authoring, with new feature that allow two or more users to work simultaneously on the same document without screwing each other up. In Word, for example, you can edit one paragraph while a collaborator is editing the next. The names of active collaborators appear on the bottom of the screen; you can click on them to send IMs or e-mails.
• Compatibility. “This is the most compatible version yet shipped,” claimed senior marketing manager Amanda Lefebvre, although the apps haven’t actually shipped and apparently aren’t ready to be demoed. The emphasis, however, seems to be on compatibility with the Windows versions of Office. (The current Windows edition is called Office 2007; the new version, Office 2010, is in beta and is scheduled to be released before the end of June.)
• Cloud computing. Microsoft’s new Mac suite is designed to work smoothly with the company’s Google-like Web Apps, its cloud storage service SkyDrive (formally Windows Live Folders) and its SharePoint enterprise servers.

A few more observations:

• Entourage is out and will be replaced by Outlook for the Mac. The IT guys will be pleased that it can import PST files and Mac users will be happy to know that they can search Outlook e-mail with Spotlight and back it up with Time Machine without having to save gigabytes of spam every time they do it.
• The program is being rewritten in Cocoa, Apple’s native object-oriented programming environment, which will presumably make it less likely to crash.
• The e-mail client will support IRM — information rights management — something IT managers insist on to prevent sensitive e-mails from being printed, forwarded or otherwise shared.
• There’s a new “ribbon” across the top of each application that puts a lot of familiar looking icons in easy reach. It also takes up a lot of screen real-estate, but it can be turned off with a click.

The product is scheduled to ship in the fourth quarter of 2010. Microsoft is not saying what it will cost.

Below: The screen shots Microsoft is showing at Macworld.

[Follow Philip Elmer-DeWitt on Twitter @philiped]