iOS 5 では、選択状態と通常状態の独自の画像でセグメント コントロールをカスタマイズできます。これを試してください:
 UIImage *segmentSelected = 
[[UIImage imageNamed:@"segcontrol_sel.png"] 
    resizableImageWithCapInsets:UIEdgeInsetsMake(0, 15, 0, 15)];
 UIImage *segmentUnselected = 
[[UIImage imageNamed:@"segcontrol_uns.png"] 
    resizableImageWithCapInsets:UIEdgeInsetsMake(0, 15, 0, 15)];
 UIImage *segmentSelectedUnselected = 
[UIImage imageNamed:@"segcontrol_sel-uns.png"];
  UIImage *segUnselectedSelected = 
[UIImage imageNamed:@"segcontrol_uns-sel.png"];
UIImage *segmentUnselectedUnselected = 
[UIImage imageNamed:@"segcontrol_uns-uns.png"];
[[UISegmentedControl appearance] setBackgroundImage:segmentUnselected 
forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
  [[UISegmentedControl appearance] setBackgroundImage:segmentSelected 
forState:UIControlStateSelected barMetrics:UIBarMetricsDefault];
     [[UISegmentedControl appearance] setDividerImage:segmentUnselectedUnselected 
forLeftSegmentState:UIControlStateNormal 
rightSegmentState:UIControlStateNormal 
barMetrics:UIBarMetricsDefault];
 [[UISegmentedControl appearance] setDividerImage:segmentSelectedUnselected 
forLeftSegmentState:UIControlStateSelected 
rightSegmentState:UIControlStateNormal 
barMetrics:UIBarMetricsDefault];
  [[UISegmentedControl appearance] 
setDividerImage:segUnselectedSelected 
forLeftSegmentState:UIControlStateNormal 
rightSegmentState:UIControlStateSelected 
barMetrics:UIBarMetricsDefault];
また、 http: //www.raywenderlich.com/4344/user-interface-customization-in-ios-5 で完全な UI カスタマイズを確認することもでき               ます。