So, a ranged basic attack is specifically described as something that can be done at-will. Dilettante lets you gain an at-will of another class but treated as an encounter. How does that work with Dragonfrost or Acid Orb. Can my warlock use one of these as a ranged basic attack thereby not expending the power as an encounter?
Basic attacks are specific attack powers that every character has, not general categories of powers.
Acid Orb and Dragonfrost count as ranged basic attacks, which means you can use them whenever an effect (usually from a warlord or other leader) allows you to make a basic attack. Dilettante says they are gained as encounter powers, which means they can only be used once per encounter (unless you recharge them somehow).
Note that the paragon tier feat Versatile Master allows half elves to use their Dilettante power at-will, so if you can wait until level 11 then you can pick any at-will power and use it at-will.