Gartner is almost as suspicious of Microsoft’s week-old interoperability promises as European Commission antitrust chief Neelie Kroes.

It’s warning open source developers to beware the patent trap.

“Do not use Microsoft’s documentation,” it says, “unless you have rigorous processes to keep track of applicable patents. Keep work that depends on these patents separate from other OSS work to avoid exposing downstream distributors and user to litigation risk.”

Last week when the tiger changed its stripes and said that it would open up the APIs and protocols in Vista, Windows Server 2008, SQL Server 2008, Office 2007, Exchange Server 2007, Office SharePoint Server 2007, and the .NET Framework, it said it would license any patents that cover this IP on reasonable and non-discriminatory terms at “low royalty rates.”

It did not define “low.”

Gartner says to ask for the RAND terms for each protocol.

The researcher expects Microsoft’s move will entice many OSS developers to port applications to Windows despite the “likely incompatibility” between RAND licensing and open source licenses like the GPL.

In fact, it’s mounting a watch for collaboration agreements between Microsoft and major OSS consortia.

It says, “This will help Microsoft expand the Windows as an organic and thriving ecosystem.”