Yes, there's a Pokedex app for iOS -- two actually, one official, another (better) one fan made. But you don't need to download either to turn an iPhone or an iPad into a Pokédex. Just ask Siri.
That's because, as VentureBeat points out, the Wolfram Alpha database just added data for all Pokémon through their first five generations -- and Wolfram Alpha is a search engine that Siri uses to answer questions. If you have one, give it a try: Just ask what type of Pokémon Squirtle is or which Pokémon has the highest rated special attack. (Data for the Generation VI Pokémon will be updated later.)
It may take speaking clearly on some of the more exotic names (or just going in and hand-typing them) but if the answer is in the database, Siri will give it back to you in her dulcet, matter-of-fact tone of voice.
Sometimes your questions will be answered with the results of a web search, but it usually pulls the information straight from Wolfram Alpha. Keep this in mind next time you're looking to settle a Poké-bet as to which Generation I Pokémon is tallest.