You'll need to create the button yourself using images. Simply create a custom UIButton and assign appropriate images for the various button states you're interested in.
You can use UIImage's 'stretchableImageWithLeftCapWidth' method to create a stretchable image from an image designed to stretch and use NSString's 'sizeWithFont' method to determine what size the button should be.
http://developer.apple.com/library/ios/documentation/uikit/reference/UIImage_Class/Reference/Reference.html#//apple_ref/occ/instm/UIImage/stretchableImageWithLeftCapWidth:topCapHeight:
Something like this might do the trick:
UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
int capHeight = 31;
int capWidth = 9;
UIImage *buttonTemplate = [UIImage imageNamed:@"button_template.png"];
UIImage *stretchedButtonImage = [buttonTemplate stretchableImageWithLeftCapWidth:capWidth topCapHeight:capHeight];
[button setBackgroundImage:stretchedButtonImage forState:UIControlStateNormal];