Hi everyone - Thanks for all your responses - you were all right! Since this popped back up, I thought I would do an update. We more or less decided not to start "serious" potty training until he seems more ready (i.e. going potty is more important than throwing a ball). We still let him sit on the potty when he wants. Some days, he goes all morning with a clean diaper (usually loses interest by the afternoon), some days he makes it very clear from breakfast that he's not really interested in the potty today. Since I've taken this attitude, he's stopped with the running away and the screaming over the poopy diapers.

Thanks again for all your help!