The next two parts of Transformers: Prime opening story arc, Darkness Rising, is now up on YouTube thanks to FanOfTransformers. Also of note it appears I might have been wrong about The Hub not posting the episodes has they now have Darkness Rising Episodes 1 and 2 available here. I assume the final three parts will be posted soon. The current episodes have a nice mention of Unicron for G1 fans, zombies, Bulkhead gets a moment to shine and more. One of the things that continue to pay off is taking a page from Beast Machines with the Vehicon cannon fodder for the Autobots to beat up resulting in more entertaining action sequences.
Transformers: Prime Episode 1 - Part 1 | Part 2
Transformers: Prime Episode 2 - Part 1 | Part 2
Transformers: Prime Episode 3 - Part 1 | Part 2
Transformers: Prime Episode 4 - Part 1 | Part 2
Hubworld Special - Part 1 | Part 2 | Behind Scenes