The baudrate setting of lpc21isp 1.31 doesn't work on OS X, making programming painfully slow. This is fixed in the lpc21isp beta 133 (from the lpc21isp Yahoo group). You can download an executable here (copy to /usr/local/arm/bin/lpc12isp): http://www.mikrocontroller.net/download/lpc21isp-133-osx