Secret hint: Equip SoW in Rawr and watch it's value skyrocket. It doesn't look like it properly calculates it's value when it's not equipped.
That can sometimes be the case if you're using incremental optimizations without simple stacking. In that situation when comparing gear Rawr won't consider significant changes in burn/conserve/cooldown stacking. Since SoW can cause a significant change in mana consumption it can require a big change in the burn/conserve so it'll only be forced to completely recompute everything once you equip it.
If you have simple stacking enabled (which I would recommend for arcane), then incremental optimization setting is ignored mostly as the whole stacking is determined dynamically for each comparison. So in this situation you wouldn't see this kind of behavior from the comparison charts.
And no, SoW is not BIS, at least based on the tests I've done. If you've run the tests some time ago you should know that simc for a long time didn't take multipliers into effect for damage proc trinkets such as mana adept and arcane power, so that may be why you got a wrong result. (actually this was only just fixed on 2/2/2012 so it's not even in the live build yet)
As for me, considering my gear, Shard of Woe is still BiS when I am playing Arcane (even though I have the chance to possess every DS Normal Caster Trinket) , and since there was a noticeable difference with the others trinkets available, I assumed it was generally the same.
I would support the idea of testing everything by yourself !