Why do I get a "__psynch_cvwait" message on Xcode?

Hi,

I am developping for iPhone. My version of Unity is 3.5.6f4. When I build and run my app on iphone or ipad through Xcode, the app receives some "__psynch_cvwait" messages that stops the main thread. Sometimes I can make the app go on, sometimes I can't and I have to force quit Xcode. When the app launches directly from the device, everything is fine. Those messages occur on a WebClient.DownloadDataAsync call.

Do you have an idea what causes that?

thx