this is NOT mine actually. the original source can be found at the end of this post. i post here just for helping people who may need it. so they don’t need to spend whole day for “googling” this like i did. 😀

the key to get the transparency is this line:

CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast;

now, this is the complete method. NOTE: this is for landscape mode (480 x 320), if you want in portrait, simply modify the size.

- (UIImage*) getGLScreenshot {
    NSInteger myDataLength = 480 * 320 * 4;

    // allocate array and read pixels into it.
    GLubyte *buffer = (GLubyte *) malloc(myDataLength);
    glReadPixels(0, 0, 480, 320, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    // gl renders "upside down" so swap top to bottom into new array.
    // there's gotta be a better way, but this works.
    GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
    for(int y = 0; y <320; y++)
    {
        for(int x = 0; x < 480* 4; x++)
        {
            buffer2[(319 - y) * 480 * 4 + x] = buffer[y * 4 * 480 + x];
        }
    }

    // make data provider with data.
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL,
								buffer2, myDataLength, NULL);

    // prep the ingredients
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * 480;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();

	/***************** to handle the transparency/alpha ******************/
	CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast;
	//CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
	/*********************************************************************/

    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

    // make the cgimage
    CGImageRef imageRef = CGImageCreate(480, 320, bitsPerComponent,
						bitsPerPixel, bytesPerRow, colorSpaceRef,
						bitmapInfo, provider, NULL, NO, renderingIntent);

    // then make the uiimage from that
    UIImage *myImage = [UIImage imageWithCGImage:imageRef];
    return myImage;
}

Source: www.iphonedevsdk.com