The situation is similar to wesdfgfgd's question MTG Gatecrash Simic Strategies for a Prerelease event
I'm a habitual Dimir player. In Gatecrash, many Dimir cards are strongly themed towards their millstone strategy or generating card advantage.
In sealed deck, and to a large extent in draft, millstone wins seem hard to pull off. Many of the capstone cards for this approach are rare, so I probably won't have them. (Milling is hard in general, and the format makes this kind of one-method-to-win focus inherently unreliable. To win on turn ten, I'd have to average milling 2.3 cards per turn - while defending myself.)
While many Dimir creatures are rather nice for reasons of evasion or card advantage, I'll hardly win with a direct creature approach against Boros or Gruul.
Looking through the non-rare spells with Cipher, most are two or 4-drops, with a few areas of effect. Most either generate card advantage (Last Thoughts) or fend for time (Hands of Binding, Voidwalk). With 18 uncommons I can hardly count on getting enough Call of the Nightwing to token flood the ciphers through, and that's the most aggressive card. (I don't feel Shadow Slice compares particularly well to a 5-drop creature in a sealed deck format.)
So, can the basic milling approach be made to work? And if not, where should I be going for the win? (And which splash colours would help me achieve it? That decision depends on my draws, obviously, but of the obvious white-splash Orzhov or green-splash Simic, what would you recommend?)
(EDIT: This question originally referred to the Gatecrash prerelease, but has been edited to be less time-dependant. The focus is primarily on sealed, but most answers are probably just as relevant to draft format; answers discussing the differences are particularly welcome.)