Microsoft has sent out press invitations to a secret announcement at 8pm tonight, continuing the company’s month long avalanche of announcements

The event is occurring via webcast at noon San Francisco time (8pm UK time). Microsoft has made no indication on what the announcement will concern, but it is expected to concern the only unannounced (and key) part of the Microsoft’s next-gen Windows 8 portfolio, Office.

The company has made several key generational-shift announcements concerning Windows 8 in the last month. Firstly it announced that it will be producing its own range of tablets called ‘Surface‘ (split into x86 and ARM based versions) and a new generation of its smartphone OS, Windows Phone 8 (which integrates with Windows 8/RT).

Microsoft has also previously made big announcements concerning Windows Server 8 and its cloud storage Skydrive, leaving just Office short of the ‘Metro’ UI treatment, which is expected to be shown off tonight.

Companies such as HP have already announced that they won’t be supporting the ARM based version of Windows 8. Office will be a key driver for Microsoft to keep hardware vendors onside.

Given the underwhelming response to the previously mentioned events (Microstrategy’s CEO described the Surface announcement as ‘disastrous‘), Microsoft has positioned Office as its key differentiator for its tablet and smartphone platforms, so how it functions under a touch screen interface is of keen interest – especially to enterprise users.

We don’t know yet whether Microsoft will show off the x86 or ARM chip version, or even whether the versions will operate or look the same. We also don’t know how the x86 version will work on desktops vs tablets. The ARM/Windows RT version is expected to be a ‘dumbed down’ version, and will be installed free on all Windows RT devices. Microsoft has struggled in the past to produce clean, simply versions of its products which are vital for success on tablets.

Apple’s iPad, for example, already offers a gamut of word processing apps, from the free to the low cost, (such as Apple Pages – £6.99) through to higher end offerings such as Quickoffice Pro for around £13.99. These price points are troublesome for Microsoft, who has been used to charging circa £150 for its Office software on desktops and laptops.

Whether Microsoft ditches the controversial ‘ribbon’ interface and goes for a simpler tiled interface will be of interest – especially where all its complex settings, buried under hundreds of tabs and interlocking feature sets will fit.

Expect a tech demo and the announcement of a consumer preview release of Office 15, which falls broadly in line strategically with its rollout of the Windows 8 consumer preview in February, which followed the developer editions.