We have two snapshots of the Protoss and Terran tech trees, taken at BlizzCon. These are obviously bound to change and change again, but it is interesting to see where things stand at the moment. Not surprisingly, the buildings and tech trees are relatively similar to those of StarCraft 1, but with a few great twists.
Protoss:

The Nexus gives access to the Vespene Gas Assimilator and the Pylon. It also makes the
Forge available, which allows Probes to warp-in
Phase Cannons.
The
Gateway is still the main unit-producing building; it allows
Zealots,
Stalkers,
Statis Orbs,
Immortals,
Templars and
Dark Templars to be warped-in. Only the Zealot can be built initially - other units require more advanced buildings.
The Gateway itself can be upgraded to a
Warp Gate, which allows the warp-in of Gateway units to any place the Protoss player has power (Pylons, Phase Prisms) at.
After getting a Gateway, a
Cybernetics Core can be warped-in, which unlocks the
Stalker and
Statis Orb. This means that
the early game Protoss forces will be comprised of Zealots, Stalkers and Statis Orbs, as opposed to StarCraft 1’s Zealots and Dragoons.
At this point in the game, the player gets to make a real decision about the direction of his build, as three options are available:
1)
StarGate. Immediately allows warp-in of
Phoenix and
Warp Ray, and can also warp-in
Carriers and a
Mothership when the
Fleet Beacon is available.
2)
Robotics Facility. This building can warp-in 3 units:
Phase Prism,
Observer (which requires the
Null Circuit building) and
Colossus (which requires the
Robotic Support Bay building).
3)
Twilight Council. Unlocks
Immortals, and leads to two more options:
Dark Obelisk, which unlocks
Dark Templars, and
Templar Archive, which unlocks
High Templars. This will likely become the default path, as it permits the player to create the most well-rounded army, supported both by solid fighting units, specialized cloaked units, and the famous Protoss caster - the High Templar.
Twilight Archons, created by merging any two Templars, are just the icing on this cake.
Terran:

The Command Center can immediately be upgraded to either a
Surveillance Station or to a
Planetary Fortress. This upgrade requires a lot of Vespene Gas, however, so it’s not actually available right at the beginning of the game.
Aside from the Supply Depot and Vespene Gas Refinery, the Command Center also gives access to the
Engineering Bay, which allows SCVs to build
Missiles Turrets, and
Sensor Domes which can be upgraded to
Radar Domes.
The Terran are unique in that the three unit producing building can have add-ons attached to them - either the
Tech Lab, which unlocks more advanced units, or the
Reactor, which effectively doubles the building’s unit producing rate.
The first unit producing building available is the
Barracks, which trains
Marines,
Medics and
Ghosts, though only the Marines are immediately available. The Tech Lab unlocks the Medics.
The
Bunker is now unlocked and can be built by SCVs.

The Barracks opens two paths for the players to choose from:
1)
Merc Haven. This is where the player recruits
Reapers. These are not trained normally - the building slowly “generates” Reapers, which can then be quickly recruited (for the right cost).
2)
Factory. This is where the Terran build their armored, mechanized units: The
Siege Tank,
Viking and
Cobra. Only the Viking is immediately available - the Siege Tank requires a Tech Lab.
The Factory opens up two more paths:
1)
Armory. This building unlocks the Cobra, and allows construction of the
Munitions Depot, which provides the SCVs with the option of constructing a
Thor.
2)
Starport.
Predator and
Dropship are immediately available, while the
Nomad and
Banshee require a Tech Lab add-on.
Battlecrusiers can be built once the
Deep Space Relay building is constructed.
The Starport itself can be upgraded to a
Star Base, which can lift off and still produce units. However, since add-ons stay on the ground, it is not clear at this point whether the Star Base will be able to produce Banshees and Nomads when airborne.
The new Terran Tech Tree allows the Terran player more options in his build than in StarCraft 1. Ghosts no longer require a full tech tree, with a Starport, Science Facility and an add on. The Thor, the supposed Terran “super unit”, can be acquired relatively early on, especially since the Armory will likely have necessary upgrades for other Terran units. Only the Battlecruiser, the Terran capital ship, requires almost every piece of the tech tree to be built.
It seems that Blizzard is attempting to make every multiplayer match of StarCraft even more diverse and unique than it was before, with more options to choose from and with more strategies to try, instead of having to follow a linear path of technology.
From the Starcraft 2 Blog.