I tried doing that, and I got a button with the OpenStep appearance, not Aqua. Here's the source code for the AppController:
- (void) awakeFromNib
{
NSRect frame = {{50.0, 50.0}, {100.0, 20.0}};
NSButton *myButton = [[[NSButton alloc] initWithFrame: frame] autorelease];
[[mainWindow contentView] addSubview: myButton];
[myButton setTitle: @"OK"];
}