Invisible Stalker (Innistrad)
With Gatecrash on the horizon, this hexproof unblockable 1/1 creature has been steadily rising in price since the Cipher keyword was released. My prediction is that Invisible Stalker will be well over $3 by the pre-release date. They are just under $1.50 now, so I would get a playset before the prices climb any higher.
Latch Seeker (Avacyn Restored)
Latch Seeker is another unblockable blue creature, good for driving home those Ciphered spells. With only 1 toughness and no hexproof to protect it, it might not last long, but if you can get a few Ciper spells off with it before it is killed, it's definitely worth putting in your deck. Latch seeker is currently riding at about 18 cents. I don't see the price moving much, so no rush on this one.
Tormented Soul (M13)
This is a good turn 1 play. It's not very intimidating at 1/1, but when you cast a cipher on it (say, hands of binding T2), then it becomes scary very quickly.
Jace, Memory Adept (M13)
Since Dimir has a mill-based aspect to it, you may want to buy 2-4 of these. This card is nigh-on essential for any respectable mill deck. If you can protect Jace for 6 turns, he will mill 60 cards right off the top of your opponent's deck... GG.
Jace's Phantasm (M13)
This guy is a 1/1 for 1 to start out, but as soon as your opponent's graveyard hits 10 cards, he becomes a big ol' 5/5. It fits the playstyle of the deck perfectly, and is a decent 1st turn drop if your deck can quickly mill some cards.
Mind Sculpt (M13)
T1: Jace's Phantasm, T2: Mind Sculpt... And you're already over halfway there!
Havengul Lich (Dark Ascension)
Havengul Lich lets you cast creatures from your opponent's graveyard. This only works if your opponent shares a color (or 2) with you. Fortunately, that problem is easily remedied with Chromatic Lantern.
Talrand, Sky Summoner (M13)
Last but not least, it's Talrand! This card has great synergy with the Ciper spells. Every time you hit with a creature with a Cipher attached, you get to cast both the Ciphered spell AND you get a 2/2 drake. It doesn't get much better than that.