Keep in mind that while UDP doesn't guarantee packet delivery, network equipment doesn't intentionally drop packets just to punish application users who chose to use UDP over a reliable TCP connection. So part of the answer is that while UDP itself is not reliable, almost all packets still make it across and in intranet environment, where one company controls all traffic and equipment UDP is virtually as reliable as TCP.
That said, one of the most popular video streaming protocols that is based on UDP/IP is RTP. RTP is defined to be application layer protocol but the specification does dictate that it is up to the receiver to monitor packet sequence numbers (part of RTP header) and make sure that the data is played out in correct sequence.
That said, there are plenty of implementations which ignore this little bit of information and they still display video with relatively good quality. Yeah, there are some glitches here and there but for the most part you can't tell as this stuff doesn't happen as frequently as some networking books would lead you to believe. At least that's my experience, but I've been working mostly on video products in corporate environments, not the Internet. I'll see if I have a different answer for you in about 6-12 months as we are moving more towards mobile/internet options.