I've actually drawn this exact shape before (rounded rectangle with a pointing triangle at the bottom). The Quartz drawing code that I used is as follows:
CGRect currentFrame = self.bounds;
CGContextSetLineJoin(context, kCGLineJoinRound);
CGContextSetLineWidth(context, strokeWidth);
CGContextSetStrokeColorWithColor(context, [MyPopupLayer popupBorderColor]);
CGContextSetFillColorWithColor(context, [MyPopupLayer popupBackgroundColor]);
// Draw and fill the bubble
CGContextBeginPath(context);
CGContextMoveToPoint(context, borderRadius + strokeWidth + 0.5f, strokeWidth + HEIGHTOFPOPUPTRIANGLE + 0.5f);
CGContextAddLineToPoint(context, round(currentFrame.size.width / 2.0f - WIDTHOFPOPUPTRIANGLE / 2.0f) + 0.5f, HEIGHTOFPOPUPTRIANGLE + strokeWidth + 0.5f);
CGContextAddLineToPoint(context, round(currentFrame.size.width / 2.0f) + 0.5f, strokeWidth + 0.5f);
CGContextAddLineToPoint(context, round(currentFrame.size.width / 2.0f + WIDTHOFPOPUPTRIANGLE / 2.0f) + 0.5f, HEIGHTOFPOPUPTRIANGLE + strokeWidth + 0.5f);
CGContextAddArcToPoint(context, currentFrame.size.width - strokeWidth - 0.5f, strokeWidth + HEIGHTOFPOPUPTRIANGLE + 0.5f, currentFrame.size.width - strokeWidth - 0.5f, currentFrame.size.height - strokeWidth - 0.5f, borderRadius - strokeWidth);
CGContextAddArcToPoint(context, currentFrame.size.width - strokeWidth - 0.5f, currentFrame.size.height - strokeWidth - 0.5f, round(currentFrame.size.width / 2.0f + WIDTHOFPOPUPTRIANGLE / 2.0f) - strokeWidth + 0.5f, currentFrame.size.height - strokeWidth - 0.5f, borderRadius - strokeWidth);
CGContextAddArcToPoint(context, strokeWidth + 0.5f, currentFrame.size.height - strokeWidth - 0.5f, strokeWidth + 0.5f, HEIGHTOFPOPUPTRIANGLE + strokeWidth + 0.5f, borderRadius - strokeWidth);
CGContextAddArcToPoint(context, strokeWidth + 0.5f, strokeWidth + HEIGHTOFPOPUPTRIANGLE + 0.5f, currentFrame.size.width - strokeWidth - 0.5f, HEIGHTOFPOPUPTRIANGLE + strokeWidth + 0.5f, borderRadius - strokeWidth);
CGContextClosePath(context);
CGContextDrawPath(context, kCGPathFillStroke);
// Draw a clipping path for the fill
CGContextBeginPath(context);
CGContextMoveToPoint(context, borderRadius + strokeWidth + 0.5f, round((currentFrame.size.height + HEIGHTOFPOPUPTRIANGLE) * 0.50f) + 0.5f);
CGContextAddArcToPoint(context, currentFrame.size.width - strokeWidth - 0.5f, round((currentFrame.size.height + HEIGHTOFPOPUPTRIANGLE) * 0.50f) + 0.5f, currentFrame.size.width - strokeWidth - 0.5f, currentFrame.size.height - strokeWidth - 0.5f, borderRadius - strokeWidth);
CGContextAddArcToPoint(context, currentFrame.size.width - strokeWidth - 0.5f, currentFrame.size.height - strokeWidth - 0.5f, round(currentFrame.size.width / 2.0f + WIDTHOFPOPUPTRIANGLE / 2.0f) - strokeWidth + 0.5f, currentFrame.size.height - strokeWidth - 0.5f, borderRadius - strokeWidth);
CGContextAddArcToPoint(context, strokeWidth + 0.5f, currentFrame.size.height - strokeWidth - 0.5f, strokeWidth + 0.5f, HEIGHTOFPOPUPTRIANGLE + strokeWidth + 0.5f, borderRadius - strokeWidth);
CGContextAddArcToPoint(context, strokeWidth + 0.5f, round((currentFrame.size.height + HEIGHTOFPOPUPTRIANGLE) * 0.50f) + 0.5f, currentFrame.size.width - strokeWidth - 0.5f, round((currentFrame.size.height + HEIGHTOFPOPUPTRIANGLE) * 0.50f) + 0.5f, borderRadius - strokeWidth);
CGContextClosePath(context);
CGContextClip(context);
The clipping path at the end can be left out if you're not going to use a gradient or some other more fill that's more complex than a simple color.