Summoning sickness is governed by CR 302.6: a creature cannot attack (or use tap/untap activated abilities) unless it has been continuously under its controller's control since the beginning of that player's most recent turn. Simply untapping the creature does not satisfy this condition.
The key point is that the rule tracks time under your control, not the creature's tapped or untapped state. Untapping is a change of state, but it has no bearing on how long you have controlled the creature. CR 509.1a confirms that when you declare attackers, each attacking creature must have been under your control continuously since your turn began (unless it has haste).
The only ways to let a creature attack despite summoning sickness are: (1) it gains Haste (CR 702.10), which explicitly overrides the summoning-sickness restriction, or (2) you simply wait until your next turn begins with the creature already under your control.
Concrete example: Your opponent controls a tapped Grizzly Bears. You cast Act of Treason, gaining control of it, and your opponent responds with Twiddle to untap it. Even though Grizzly Bears is now untapped and under your control, it still has summoning sickness — you did not control it at the start of your turn — so you cannot attack with it this turn unless it has haste.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.