Are all 151 pokémon catchable on the same account?

As it appears right now it is possible to get 146, all of which you can catch or evolve up. The 5 legendary (Articuno, Zapdos, Moltres, Mew, Mewtwo) are uncatchable at least at this time. Whether they are added later or will be rewards for something special is unknown.

Keep in mind that the following information has been extracted from the Pokemon Go source code, and while its accuracy may seem feasible and reasonable, it is not yet official. That being said, this is what most people seem to believe at this point...

The following a list of the base capture rate for each of the 151 Pokemon in Pokemon Go (source):

• Base Capture Rate: List of Pokemon
• 56%: Magikarp
• 48%: Oddish
• 40%: Caterpie, Weedle, Pidgey, Rattata, Spearow, Ekans, Sandshrew, Nidoran (male), Nidoran (female), Jigglypuff, Zubat, Venonat, Diglett, Meowth, Psyduck, Mankey, Poliwag, Abra, Machop, Bellsprout, Tentacool, Geodude, Slowpoke, Magnemite, Doduo, Seel, Grimer, Shellder, Drowzee, Krabby, Voltorb, Exeggcute, Koffing, Rhyhorn, Horsea, Goldeen, Staryu
• 32%: Paras, Ponyta, Gastly, Cubone, Tangela, Eevee, Porygon, Omanyte, Kabuto, Dratini
• 24%: Clefairy, Vulpix, Growlithe, Farfetch'd, Mr. Mime, Scyther, Jynx, Electabuzz, Magmar, Pinsir, Tauros, Gloom
• 20%: Metapod, Kakuna, Pidgeotto, Nidorina, Nidorino, Poliwhirl, Kadabra, Machoke, Weepinbell, Graveler
• 16%: Bulbasaur, Charmander, Squirtle, Pikachu, Hitmonlee, Hitmonchan, Lickitung, Chansey, Lapras, Ditto, Aerodactyl, Snorlax, Kangaskhan, Onix, Raticate, Fearow, Arbok, Sandslash, Wigglytuff, Golbat, Parasect, Venomoth, Dugtrio, Persian, Golduck, Primeape, Tentacruel, Slowbro, Magneton, Dodrio, Dewgong, Muk, Cloyster, Haunter, Hypno, Kingler, Electrode, Exeggutor, Weezing, Rhydon, Seadra, Seaking, Starmie
• 12%: Rapidash, Marowak, Vaporeon, Jolteon, Flareon, Omastar, Kabutops, Vileplume
• 10%: Butterfree, Beedrill, Pidgeot, Nidoqueen, Nidoking, Poliwrath, Alakazam, Machamp, Victreebel, Golem
• 8%: Ivysaur, Charmeleon, Wartortle, Raichu, Clefable, Ninetales, Arcanine, Gyarados, Dragonair, Gengar
• 4%: Venusaur, Charizard, Blastoise, Dragonite
• 0%: Articuno, Zapdos, Moltres, Mewtwo, Mew

Keep in mind that base capture rate means the probability or likelihood that you will catch a Pokemon with a PokeBall, with no throw bonus, curveball, Razz Berry, or any other feature that increases or decreases your likelihood. You can read here about what factors can alter catch rate.

So to answer your question, no, not all 151 Pokemon are catchable on one account, or on any sum of accounts, for that matter. However, all 151 are indeed attainable on one account, but as it stands currently, they would not all be attained by catching.

From the above statistics, you can see that there are five Pokemon that are currently not catchable (with a base capture rate of 0%), including Articuno, Zapdos, Moltres, Mewtwo, and Mew. There is speculation that these Pokemon will be "given away" at Niantic events. It is also interesting to note that the ever mysterious Ditto has a base catch rate of 16%, so if anyone can find it, it's not uncatchable.

Here is a helpful video that essentially summarizes all of the above information in a visual, easy to understand fashion.

Pokémon, which are not in the game yet:

1. Ditto
2. Mew
3. Mewtwo
4. Zapdos
5. Moltres
6. Articuno

151 Kanto Pokémon - 6 = 145 catchable Pokémon

However the missing ones exist in the source files of the App: Also read at this article.