Using Mono 2.2.1 and Visual Studio 2010 simultaneously

I created a C# .NET 3.5 project with Visual Studio 2010, add some classes and forms and close it. After that I open the project with Monodevelop 2.2.1 and add some additional stuff and close ist again. Now when I reopen the project with Visual Studio 2010, the project conversion manager pops up and wants to convert the project. I analysed the project files and the differences are only the following two lines:

Visual Studio 2010:

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
...
    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
...

Monodevelop 2.2.1:

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
...
    (missing)
...

Do you have any suggestions how to avoid this? Like how to tweak Monodevelop to write the project files Visual Studio wants to read or disable the conversion agent or converting silently in the background.

And please no complaints about the tool chain ;) It must be like that.