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I have had multiple discussions regarding "at will" abilities. From Paladin players who would like to Detect Evil as an immediate or free action (followed by the requisite move action concentration) to one gamemaster who decided for his last encounter used all four of his creatures "at will" abilities as free actions before taking it's swift, move and then standard.
I tend to consider "at will" abilities which "function like x spell" to have a casting time and be noticed as if one were casting a spell of an equivalent nature. However, this would tend to hamstring Paladin in battle causing them to loose a standard action along with the move for concentration and for that matter one trying to covertly assess the trustworthiness of an NPC. I am adverse to hamstringing my PC's. At the same time, the Druid Wild Shape ability is very explicit stating the differences in the ability and Beast Shape I regarding ability activation time etc.
The issue with the character class ability is also complicated by monsters who generate spell effects "at will". With the exception of the gamemaster cited, most gamemasters I have run into are fair and consistent. The thing that troubles me though is most gamemasters I have played with tend to use "at will" abilities as though the creature causing the effect is generating the effect both still and silent. There is no perception check to notice casting/point of effect generation. At times, I have played where the source has remained concealed whilst all of a sudden, the party is facing a summoned creature/spell effect out of thin air for single or multiple rounds running.
Usually at worst I give my players a perception check to notice the use of an "at will" power when a spell is being emulated. For verbal spells I use a DC 0 plus distance and circumstance modifiers (equivalent to hearing the details of a conversation). I choose this a for having to "speak in a strong voice" during the typical spell casting which the ability emulates. I tend toward a DC 15 for somatic plus distance and circumstance modifiers (difficulty to notice an average concealed door). This presumes the party is unawares of the creature(s) presence.
Personally, if I think it will come up, before I sit at the table I ask for gamemaster clarification. I would, however, like to be directed to either an official ruling on this or be provided with one. That way if it ever comes up in a society game I can adjudicate the rule and know what to expect consistently and objectively per the rules.
It depends on the ability: An "at-will" SU ability is different from an "At-will" EX ability or an "at-will" SL ability. They each follow the general rules for that ability type unless specifically overridden by rules in that ability itself. See the "Actions in Combat" table. You'll see that all three types are listed under "standard action", but they each provoke differently.
The "at-will" doesn't tell us what action (if any) is required to use the ability. It only tells us that there is no "uses-per-day" or similar restriction.
Using an extraordinary ability is usually not an action because most extraordinary abilities automatically happen in a reactive fashion. Those extraordinary abilities that are actions are standard actions unless otherwise noted.
A spell-like ability has no verbal, somatic, or material component, nor does it require a focus or have an XP cost. The user activates it mentally. A spell-like ability takes the same amount of time to complete as the spell that it mimics (usually 1 standard action) unless otherwise stated.
Using a supernatural ability is a standard action unless noted otherwise.
At will is not the same as Constant. For example casters can use detect magic at will. However, noble Drow have detect magic as constant. The latter effectively means they have it on all the time. The former means they have to cast it.
Using an extraordinary ability is usually not an action because most extraordinary abilities automatically happen in a reactive fashion. Those extraordinary abilities that are actions are standard actions unless otherwise noted.
A spell-like ability has no verbal, somatic, or material component, nor does it require a focus or have an XP cost. The user activates it mentally. A spell-like ability takes the same amount of time to complete as the spell that it mimics (usually 1 standard action) unless otherwise stated.
Using a supernatural ability is a standard action unless noted otherwise.
Emphasising this post because it contains the relevant rules information for you. At-will means nothing for the action as stated by people in the thread: the rules text tells you, and if it doesn't specify, you follow the logic above.