iphone dev: networkActivityIndicator works well on iPad but not on iPhone

Question

I have a Tab-based application and this problem:

I load my application showing a simple info view, then when I click on another tab I'd like to show networkActivityIndicator (because I make a web request) using

[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

in loadView method of my ViewControllers (that manage waht I selected on tab bar), then I hide the networkActivityIndicator in viewDidAppear method

the problem is that on iPad it works as I expected, showing it when I click the tab until the view is loaded, but on iPhone not , if I'm lucky it appears just 1 second after the view has shown!!! I tried on both device and simulator.

Maybe it depends on OS version, iPad 3.2 vs iPhone 4.0 (or 4.0.2) but, maybe some bug in new OS 4???