The OpenMote developers also were surprised at the low signal strength I reported in my last post: -69 dBm at 25 cm separation. It wasn't clear which of the two motes was malfunctioning, so they replaced both of them. Thanks, guys!
It turns out that only one of the original batch of motes was causing the trouble. The other mote and the replacements all report a signal strength around -40 dBm at 25 cm.