Last week we were greeted with Microsoft’s invite for their event, which takes place today.  Though the invite itself was vague, just containing the date, time and venue, speculation over its product details quickly took over.

First was the rumor of a Microsoft Windows 8 tablet, as in their very own tablet not manufactured by another OEM.  Next was the Nokia Windows 8 tablet, thanks to the teaser on Nokia’s Facebook page, which makes it seem like the upcoming announcement is not solely Microsoft related, but dealing with the availability of the Nokia 808 PureView in the US.

But the latest rumor is that Microsoft is teaming up with Barnes & Noble for a tablet that would compete with Amazon’s Kindle Fire, and not the iPad.  That effort is said to be  part of Microsoft’s previous investment in Newco, Barnes & Noble’s digital arm.

The biggest clue that Microsoft will not be launching a Microsoft Windows 8 tablet is their agreement with B&N when they invested in Newco stating that subsidiaries or affiliates do “not compete, with NewCo in the business of selling or making available Reading Content or similar e-reading content, e-reader software or e-reader devices.”

The agreement means that Microsoft and Barnes & Noble would work together to create tablets and/or e-readers in the future, and that Microsoft cannot make tablets with other OEMs as it would violate their agreement with B&N.

But that still leaves other questions, as the B&N Nook currently runs on rival OS, Android 2.3 Gingerbread.  Perhaps the true nature of their partnership is to reveal an e-reader tablet for Microsoft’s own mobile OS, Windows Phone.

Another conjecture is that Microsoft would be announcing the acquisition of Yammer Inc., the social network for businesses.  Rumors about the $1.2 billion acquisition surfaced last week and it could be today that the rumors would either be put to rest or be proven true.

Whatever Microsoft’s announcement may be, we’ll just have to wait a few more hours, as the event starts at 3:30 PST.