I don't know if you even got an answer somewhere else but i wrote a little method to do this a while ago.
http://iky1e.tumblr.com/post/3054935938/save-opengl-content-to-photolibrary
-(void)saveRectangle:(SPRectangle*)rectangle{
int bufferLenght = (rectangle.width*(rectangle.height+30)*4);
int myWidth = rectangle.width;
int myHeight = rectangle.height;
int myY = self.stage.height-rectangle.y-rectangle.height;
int myX = rectangle.x;
unsigned char buffer[bufferLenght];
glReadPixels(myX, myY, myWidth, myHeight, GL_RGBA, GL_UNSIGNED_BYTE, &buffer);
CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, &buffer, bufferLenght, NULL);
CGImageRef iref = CGImageCreate(myWidth,myHeight,8,32,myWidth*4,CGColorSpaceCreateDeviceRGB(),
kCGBitmapByteOrderDefault,ref,NULL, true, kCGRenderingIntentDefault);
uint32_t* pixels = (uint32_t *)malloc(bufferLenght);
CGContextRef context = CGBitmapContextCreate(pixels, myWidth, myHeight, 8, myWidth*4, CGImageGetColorSpace(iref),
kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Big);
CGContextTranslateCTM(context, 0.0, myHeight);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextDrawImage(context, CGRectMake(0.0, 0.0, myWidth, myHeight), iref);
CGImageRef outputRef = CGBitmapContextCreateImage(context);
UIImage *image = [[UIImage alloc] initWithCGImage:outputRef];
free(pixels);
UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil);
[image release];
}