This works for a MSBuild located in the .NET folder (4.0) (i.e. no spaces in path) but when using MSBuild 12.0.

Command Line Argument (in Jenkins config):
/p:SolutionFileName=Framework.sln;.\VS2010_projects\Framework2010.sln
or
/p:SolutionFileName="Framework.sln;.\VS2010_projects\Framework2010.sln"
(this works at command line when not using cmd /c msbuild.exe ...)

Log in console:
Path To MSBuild.exe: C:\Program Files (x86)\MSBuild\12.0\bin\amd64\msbuild.exe
Executing the command cmd.exe /C "C:\Program Files (x86)\MSBuild\12.0\bin\amd64\msbuild.exe" /m /p:SolutionFileName=Framework.sln;.\VS2010_projects\Framework2010.sln ci.msbuild.xml

Error:
MSBUILD : error MSB1006: Property is not valid.
Switch: .\VS2010_projects\Framework2010.sln

Command Line Argument:
/p:SolutionFileName='"Framework.sln;.\VS2010_projects\Framework2010.sln"'

Log in console:
Path To MSBuild.exe: C:\Program Files (x86)\MSBuild\12.0\bin\amd64\msbuild.exe
Executing the command cmd.exe /C "C:\Program Files (x86)\MSBuild\12.0\bin\amd64\msbuild.exe" /m /p:SolutionFileName="Framework.sln;.\VS2010_projects\Framework2010.sln" ci.msbuild.xml

Error:
'C:\Program' is not recognized as an internal or external command,
operable program or batch file.

If "cmd /c" is executed on a command line you need to put an extra quote in front and at end to get i to execute. I cannot come up with a workaround to execute this on Jenkins, and it is valid msbuild syntax. Anyone?

Example that works on command line.
cmd.exe /C ""C:\Program Files (x86)\MSBuild\12.0\bin\amd64\msbuild.exe" /m /p:
SolutionFileName="Framework.sln;.\VS2010_projects\Framework2010.sln" ci.msbuild.xml"