Bonus to attack from hide in shadows + attack from behind in this combination. Check every 2 turns if foes can hear (see) me moving in shadows.
I like the idea of sneak not being 100% reliable. Taurondir's suggestions are good ones, although perhaps you could just have one check if 1+ enemy is within (two?) squares.
Rogue damage values, in my opinion, could be nerfed in the mid-levels. Party of three, vanguard, beacon, and rogue, and the rogue more often than not killed a majority of enemies on the field, particularly with a strong minor attack (offhand cut) and any of the choices of major (twin strike, telling blow). Once lunge came around, the beacon seemed even less relevant (until the final couple levels, when item and strength bonuses plus higher-level beacon abilities came into play).
In any case, the vanguard often got left in the dust by the rogue (using dash, of course), even though both were maximizing their main stats (strength/agility).
I'd rather the stealth stay as is it for the time being. We already have limited options in combat and it's random already, and having one of your 2/engagement powers being nullified by a totally random dice roll - and sometimes you get one move per 24 hours - so what in fact would have happened is that your Rogue just spent 3 days sneaking to a mob, just to get spotted because 5 mobs on grid each make a random roll and you got unlucky.
You could add mechanics such as "Lose Sneak if in contact with 2 mobs" or "cant Dash past opponents without losing Sneak" which is tactical, not random, and more logical game wise.