The proc info seems right, 25% chance to increase damage and 1% chance to lower damage (both of which I've experienced).
As for the effect itself, when it procs, a buff goes into your short duration (song) window called Fire of Dawn. It lasts for 1:30 and can proc in conjunction with gift of mana (as in, you can get both gift of mana and fire of dawn on the same spell). It does not fade if a spell is resisted, only if it lands. The extra cast time makes this useful for opening a fight with and then following up with one of the normal elemental nukes.
Edit: I just tested it and the duration is fixed at 1:30. I thought maybe it was 1:00 plus the 50% enhancement from AA's but it stayed fixed with more extension on.
Edit2: I was fiddling around with the max damage I could get out of the effect and so far I know I can get 4400 without a crit. I've tried putting on my epic effect at the same time but I always end up nuking for 2800-3k so I'm thinking that the epic effect takes priority over this, I'm not sure though as I may have just gotten a weak nuke with the increased damage of my epic giving me only a sub-par raise. Further data when it comes.
Edit3: After a lot of fiddling around I've discovered that our epic and this spell's effect will stack. I've managed to get up to 4.8kish without a crit as well. Stacking those two abilities is really nice when you're soloing and you just got a gift of mana.
Edited, Mon May 8 09:04:23 2006