I would say yes:
My thoughts: Flanking is the result of a tactical disadvantage for a defender. I know of no restrictions on flanking based upon the status of an attacker (other then the ones mentioned, obviously). The primary advantage for a flanking attacker is that the defender is less able to anticipate the attacks (threatened).., not the other way around. (FYI: A sneak attack versus an invisible creature is not possible, the target has to be seen.)
Note: Without pinning/tripping/etc an adversary, there’s always a chance of escape. On its’ turn, the invisible creature may, for example, try to:
- Bull-rush a flanking creature (not provoking any attacks of opportunity, for having total concealment), OR;
- Tumble (DC 25, trained only) his way out. If tumbling is successful, the invisible creature moves through the occupied area at half speed WITHOUT provoking any attacks of opportunity. If the invisible creature fails the tumble-check, it can’t move through the occupied area but still doesn’t provoke any attacks of opportunity for having total concealment (according to RAW), OR;
- Grapple, receiving a +2 on the initial grab (melee touch attack), penalties to the defenders AC apply as well. Etc.
Flanking depends on being threatened, which only mentions the ability to attack a square, it does not rely on being able to execute any attacks of opportunity, or targets, or anything else AFAIK. A square is an area in my book, not an opponent. This eliminates concealment as a factor (for the area a totally concealed occupies can still be attacked, thus is threatened), just like the differences between line of sight and line of effect, for example.
To be directly on the other side of a character who is being threatened by another character. A flanking attacker gains a +2 flanking bonus on attack rolls against the defender. A rogue can sneak attack a defender that she is flanking.
To be able to attack in melee without moving from your current space. A creature typically threatens all squares within its natural reach, even when it is not its turn to take an action. For a Medium or Small creature this usually includes all squares adjacent to its space. Larger creatures threaten more squares, while smaller creatures may not threaten any squares except their own.
My only concern is the “typically” & “usually,” I interpret this as, unless mentioned otherwise (f.e. in case of reach attacks that don’t threaten adjacent squares)