My guess would be that they either work together on some "rather open document format" since the fast track for Microsofts proprietary format did fail (6k pages of documentation with references to closed stuff like their own implementation in Word 97 is IMO proprietary) or that they work together on the successor of Java/C#.
The reasons for those:
1) MS wants to have their Office series still be used in the government, this won't happen, if they do not support a format standardized by the ISO. This means they can either work on a new format or try to improve the old OOXML format. The "long track" will need a real technical check of the whole specs, the current specs would probably not pass. So they could work together with a company already having an office document format standardized by the ISO: SUN. Sun finances a lot of the development of OpenOffice and with this they were involved in working on the ODS.
So either Sun will work with Microsoft on improving their specs or on implemting ODS direcltly in MS Office (I don't think so).
2) The other thing they could work on is some new Programming language. Sun is the company behind Java just as Microsoft is the company behind C#. With combined efforts they might want to create a real successor of Java/C# or to have both behave in a more similar way. Creating a "new" programming language takes quite some time, so if they want to have something new in maybe two years they would have to start right now...
Like I said, all of this above is just pure speculation, though most of it would make quite some sense. Okay, in some "wet dream"-scenario it could also be about offering specs for implementing DirectX in Solaris, though this is very unlikely and no idea what Sun would provide MS instead...