Does giving a creature haste allow it to attack the same turn it was stolen with Act of Treason?

Act of Treason gains control of a creature, untaps it, and gives it haste until end of turn. Haste is the key ability here: normally a creature must have been under your control since the beginning of your turn to attack or activate tap abilities (CR 302.6). Haste overrides this restriction (CR 702.10a).

Because Act of Treason itself grants haste, you don't need any additional source of haste — the card handles it all in one shot. The creature untaps (in case it was tapped), and haste lets it attack even though it just came under your control this turn.

CR 302.6 states that a creature cannot attack unless it has been under your control since the beginning of your most recent turn, unless it has haste. CR 702.10a confirms haste lets the creature ignore that 'summoning sickness' restriction entirely.

Concrete example: Your opponent controls a 5/5 that attacked last turn and is now untapped. You cast Act of Treason on your turn. It untaps (it was already untapped, so no change), gains haste, and you declare it as an attacker — dealing 5 damage to your opponent or their planeswalker that same turn. At end of turn, control reverts to your opponent.

So yes, giving a stolen creature haste (whether through Act of Treason or another source) absolutely allows it to attack the turn you steal it. Without haste, a stolen creature would be subject to summoning sickness and could not attack.