Can I give a UIToolBar a custom background in my iPhone app?

Answering my own question here!!! Overriding the drawRect function and creating an implementation of the UIToolbar does the trick :)

    @implementation UIToolbar (CustomImage)
- (void)drawRect:(CGRect)rect {
    UIImage *image = [UIImage imageNamed: @"nm010400.png"];
    [image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
}
@end

UIToolbar inherits from UIView. This just worked for me:

[topBar insertSubview:[[[UIImageView alloc] initWithImage:[UIImage imageNamed:BAR_BKG_IMG]] autorelease] atIndex:0];

Slightly modified version of loreto's answer, which works for me on ios 4 and 5:

// Set the background of a toolbar
+(void)setToolbarBack:(NSString*)bgFilename toolbar:(UIToolbar*)toolbar {   
    // Add Custom Toolbar
    UIImageView *iv = [[UIImageView alloc] initWithImage:[UIImage imageNamed:bgFilename]];
    iv.frame = CGRectMake(0, 0, toolbar.frame.size.width, toolbar.frame.size.height);
    iv.autoresizingMask = UIViewAutoresizingFlexibleWidth;
    // Add the tab bar controller's view to the window and display.
    if([[[UIDevice currentDevice] systemVersion] intValue] >= 5)
        [toolbar insertSubview:iv atIndex:1]; // iOS5 atIndex:1
    else
        [toolbar insertSubview:iv atIndex:0]; // iOS4 atIndex:0
    toolbar.backgroundColor = [UIColor clearColor];
}

This is the approach I use for iOS 4 and 5 compatibility:

if ([toolbar respondsToSelector:@selector(setBackgroundImage:forToolbarPosition:barMetrics:)]) {
    [toolbar setBackgroundImage:[UIImage imageNamed:@"toolbar-background"] forToolbarPosition:UIToolbarPositionAny barMetrics:UIBarMetricsDefault];
} else {
    [toolbar insertSubview:[[[UIImageView alloc] initWithImage:[UIImage imageNamed:@"toolbar-background"]] autorelease] atIndex:0];
}