sm/ruin is already viable. albeit only on a couple of fights in raids atm. if its a single target fight, ds/ruin will beat out by a small margin. however, fights with adds (gaar, majordomo, golemagg are the big ones that come to mind) sm/ruin actually closes that dps gap even more with the ability to corruption on adds and gain instant shadow bolt procs for whatever is main target. now outside of raids, sm/ruin is my ideal spec as it is very useful in pvp, and dungeons where the debuff limit isnt always constantly maxed out, it becomes more useful as well. for straight single target damage, ds/ruin is king atm however
this is my sm/ruin spec, http://db.vanillagaming.org/?talent#IE0ibRboVZZxx0tr0z, (some points are for pvp purposes, and can be shuffled around, imp drain life / curse of agony)
and this is my ds/ruin spec, http://db.vanillagaming.org/?talent#Ix0 ... ooZgx0tr0z (again you with some points for pvp usefullness and just in case i need to be the imp bitch) both, destructive reach and shadow burn are must haves really. many fights the added range will be a life saver, and shadowburn on cooldown weaved in between shadow bolt spam for max damage. go into raid with tons of shards, maybe even 2 soul bags if you wanna go buck wild. regardless of spec however, rotation doesnt sway too far away from > apply curse > shadow bolt > shadow bolt > shadow bolt > shadow bolt > shadow bolt > shadow bolt