setProgress is no longer updating UIProgressView since iOS 5

Question

I have a little trouble with a progress bar since iOS 5 came out. The code below was working fine before iOS 5 but with iOS 5 the progress bar is no longer displaying the new progress that is set within a loop.

The code is expected to work like this:

1. Create the progress bar (works)
2. In a new background process: Set an initial progress of 0.25 (works)
3. In the same background process: Update the progress while going thru the loop (worked in iOS 4)

Here's the code for the bar init:

// create a progress bar
UIProgressView *progressBar = [[UIProgressView alloc] initWithFrame:CGRectMake(coverSizeX*0.25, coverSizeY - 34.0, coverSizeX*0.5, 9.0)];
progressBar.progress = 0.0;
progressBar.progressViewStyle = UIProgressViewStyleBar;

and in a different thread it sets a starting point for the progress to 0.25:

// set an initial progress
[progressBar setProgress: 0.25];

a little later it is updating the progress within a loop to display the download progress:

// within a for-loop:
NSNumber *counterPercentage;
for ( pageDownload = 1; pageDownload < pagesToDownload; pageDownload++ ) {
    counterPercentage = [[NSNumber alloc] initWithFloat: (float)pageDownload / (float)((float)pagesToDownload)];
    [progressBar setProgress: [counterPercentage floatValue]];
    [progressBar performSelectorOnMainThread:@selector(setNeedsDisplay) withObject:nil waitUntilDone:YES];
}

… but the progress is not shown on the screen, the progress bar is stuck at the initial 0.25 progress that was set.

Were there any changes with the iOS 5 release that could have broken it?