I tested opencv on beagleboard for onboard image processing. The examples provides with opencv runs fine , but the performance is poor compared to a PC.
BeagleBoard (OMAP 3530, ARM Cortex-A8 600MHz, 128MB ram)
PC (Intel core2duo 2.13GHz, 1GB ram)
It took me about 4.4 sec on beagle for a simple facedetect example to detect all faces and around 0.35 sec on PC