When displaying MSBuild output, accents and special caracters are not displayed properly:
Here is a sample load.proj file :
If you run it from CMD :

C:\>C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe load.proj
Microsoft (R) Build Engine, Version 4.0.30319.1
[Microsoft .NET Framework, Version 4.0.30319.239]
Copyright (C) Microsoft Corporation 2007. Tous droits réservés.

La génération a démarré 10/01/2012 09:51:28.
Projet "C:\load.proj" sur le noud 1
(cibles par défaut).
run:
éà@u$ù
Génération du projet "C:\load.proj"
terminée (cibles par défaut).

La génération a réussi.
0 Avertissement(s)
0 Erreur(s)

Temps écoulé 00:00:00.04

Both message comming from MSBuild and message comming from my .proj are displayed properly (with éè...)
When executed from jenkins:

Path To MSBuild.exe: C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe
Executing command: cmd.exe /C C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe load.proj && exit %%ERRORLEVEL%%
[dummy test] $ cmd.exe /C C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe load.proj && exit %%ERRORLEVEL%%
Microsoft (R) Build Engine, Version 4.0.30319.1
[Microsoft .NET Framework, Version 4.0.30319.239]
Copyright (C) Microsoft Corporationÿ2007. Tous droits r‚serv‚s.

La g‚n‚ration a d‚marr‚ 10/01/2012 09:55:11.
Projet "C:\load.proj" sur le noud 1 (cibles par d‚faut).
run:
‚...@u$—
G‚n‚ration du projet "C:\load.proj" termin‚e (cibles par d‚faut).

La g‚n‚ration a r‚ussi.
0 Avertissement(s)
0 Erreur(s)

Temps ‚coul‚ 00:00:00.04
Finished: SUCCESS

You can see that "génération" is written "g‚n‚ration" with missing "é"

[EDIT] It does not seems to be related to MSBuild plugin, but rather to MSBuild
I tried a different approach and run MSBuild from a command line task inside jenkins:
[dummy test] $ cmd /c call C:\Users\xxx\AppData\Local\Temp\hudson3226420618812873918.bat

C:\>C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe load.proj
Microsoft (R) Build Engine, Version 4.0.30319.1
[Microsoft .NET Framework, Version 4.0.30319.239]
Copyright (C) Microsoft Corporationÿ2007. Tous droits r‚serv‚s.

La g‚n‚ration a d‚marr‚ 10/01/2012 10:03:35.
Projet "C:\load.proj" sur le noud 1 (cibles par d‚faut).
run:
‚...@u$—
G‚n‚ration du projet "C:\load.proj" termin‚e (cibles par d‚faut).

La g‚n‚ration a r‚ussi.
0 Avertissement(s)
0 Erreur(s)

Temps ‚coul‚ 00:00:00.04

C:\>exit 0
Finished: SUCCESS
So it is not related to MSBuild plugin as suggested in the issue : https://issues.jenkins-ci.org/browse/JENKINS-10856
But rather to MSBuild.
I also tested a simple command :
echo éè
and it works without problem.