It relates to the "skin cache" file called fm2012.skc.
You can delete the preferences and cache from the library folder, Steam does write to these foldesr. However they are hidden if you are using OSX Lion - you have to make it show by opening the terminal and typing the following
chflags nohidden ~/Library/
This will allow you to see your library and delete the caches. However the simple way around is to disable the "use skin cache" tick box in the preferences panel - this has completely resolved the problem.
You can delete the preferences and cache from the library folder, Steam does write to these foldesr. However they are hidden if you are using OSX Lion - you have to make it show by opening the terminal and typing the following
chflags nohidden ~/Library/
This will allow you to see your library and delete the caches. However the simple way around is to disable the "use skin cache" tick box in the preferences panel - this has completely resolved the problem.