I have only occasionally been giving Adam a regular cup and then usually with a straw. He still was using sippy cups most of the time. This was not really because I didn't think he could drink from a cup, but because he spills alot and it was easier for me to give him a sippy and not have a mess to clean up. So, completely my fault that we couldn't check "can drink from cup" off his developmental checklist.
But, this morning, I was feeling very tolerant(or maybe because I know I have to scrub the kitchen floor today anyway) and handed him an open cup of juice. He drank it as if he's been doing it forever, no spills.