You could also mask the component. With a bit fiddeling you can get the size of the component and cut it out with following code:
CALayer* mask = [[CALayer alloc] init];
[mask setBackgroundColor: [UIColor blackColor].CGColor];
[mask setFrame: CGRectMake(10.0f, 10.0f, 260.0f, 196.0f)];
[mask setCornerRadius: 5.0f];
[picker.layer setMask: mask];
[mask release];
Don't forget
#import <QuartzCore/QuartzCore.h>
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