This is a really interesting article on the Viddler.com architecture. They are using HDFS and Amazon S3 for storage. Amazon EC2 servers do all of the encoding work. They have experimented with Cassandra for storing dashboard positions.
Overall, their architecture is all of the place, but they are working on improving that situation.
The real takeaway is their focus on customer service:
What matters in the end is what the users sees, not the architecture. Iterate and focus on customer experience above all else. Customer service is even valued above sane or maintainable architecture. Build only what is needed. They could not have kick started this company maintaining 100% employee ownership without running ultra scrappy. They are now taking what was learned in scrappy stage and building a very resilient multi-site architecture in a top-tier facility. Their system is not the most efficient, or the prettiest, the path they took is the customer needs something so they built it. They go after what the customer needs. The way they went about selecting hardware, and building software, with an emphasis on getting the job done, is what built the company.