http://blog.mugunthkumar.com/coding/ios4-issue-nsurlconnection-and-nsoperation/
There is some bug in iOS4 when calling NSURLConnection from a thead different from the main one. To solve this issue use the following code:
//iOS 4 bug fix
if (![NSThread isMainThread]) {
[self performSelectorOnMainThread:@selector(start)
withObject:nil waitUntilDone:NO];
return;
}