I still don't use Fire Nova a whole lot on trash. In fact, AoE on the little skeletal guardian guys (the ones who explode in a hail of AoE) is pretty risky business if you've got too many lazy dps just dropping green target circles on everything and your melee spread out on targets. I'll use it if I have a glaring shortage of abilities available to use and that's about it. I don't compete for damage on trash...we've yet to encounter trash in ICC where pushing it to kill them all ASAP has been worth the hassle. Mana is largely a non issue now with 2pc T10
On bosses, however, I think Flame Shock is here to stay as our opener of choice for tier 10 and continuing through to Cataclysm. Normally, for the sake of a few hundred extra damage over the course of 18 seconds I wouldn't care. At all. It's an added complication and another thing to track that offers a minor boost and since I don't run the level of content that demands every last fraction of an ounce of performance, Flame Shock on its own is a "take it or leave it" kind of thing.
With the 264 relic totem, however, that changes. I'm currently using the 245 "elemental" relic totem (chance to grant 200 haste on LB cast) which is nice, but it's not 100% uptime and it has me feeling reluctant to use CL on boss fights even if they involve adds (I do use CL where appropriate, but it bothers me that I'm missing out on the chance for 8% melee haste when I do it. The 264 "elemental" totem, on the other hand, offers a 220 haste bonus. It takes the first full cycle of Flame Shock periodic ticks to build up to that point, but then once you've got it you only have to use Flame Shock once every 27 seconds to maintain it. Given that haste is a superior choice for enhancement shaman over AP in addition to the fact that you can re-apply Flame Shock from range means that you've got a better chance to preserve the benefit than you would with the "enhancement" relic on fights where you might not be able to maintain melee range at all times. (Marrowgar would be a decent exammple of this; now that he's tauntable, if he decides to load up his Bonestorm at a bad time relative to relic totem procs, you can still tag him on the way by and not have to worry about it).
With that in mind, I plan on working FS into my rotation now so that it's second nature to me to keep the debuff ticking when it comes time to change relic totems. That's a ways off though...for all the talk I do about haste rating being > AP, the 4pc T10 bonus is still more attractive to me than an extra 20 haste rating with slightly better uptime :P