From what I've read, the ultra pasteurized milk in other countries is somewhat different, sold in juice-like cartons, and has an even longer shelf life, up to 9 months, no refrigeration required until open. It is popular in areas where refrigeration is difficult.
However, the US practice appears to be different. Here, I've read it has a shelf life of 1-3 months, and is typically labeled 4-6 weeks out, and labeled 'requires refrigeration.' The carton I saw looked exactly like a half gallon milk carton, except for the large font "ultra pasteurized" label on the front of the carton. Since I didn't know about UHT, it was the date that alerted me.
I didn't read the fine print to see about use after opening, but I would expect it to have recommendations similar to regular milk. But if it doesn't sour the same way, or curdle, it's more likely to be used long past expiration or recommended use date. That's scary enough, but the article I saw suggested a greater concern, that although required by law to be labled ultra, a processor might hold the milk until the usual 3 week expiration, then put it in regular cartons and nobody would be the wiser.
Being around dairy animals as a kid, I know how quickly 'real' milk curdles. I remember my sister visited once, and forgot a baby bottle in the back of her car, in the summer. Three days later when it was discovered, her store-bought milk hadn't curdled. Scary.
Ultra pasteurized milk is being touted as safer, kills all the bacteria. But I wonder if drinking it makes people less resistant to normal bacteria, like using anti-bacterial soap all the time.