The troubleshooting tips from the Apple knowledgebase article USB and FireWire Quick Assist did not provide the solution. However, one of them made me think there might be another type of device causing a conflict. I got my iMac, running Snow Leopard 10.6.5, to recognise my USB connected HP Photosmart C7280 by removing two unconnected cell/mobile phones from my computer's list of paired Bluetooth devices.
[crarko adds: I haven't tested this one. The relationship between USB and Bluetooth is something to be on the lookout for when troubleshooting peripheral problems in general.]

