This is an interesting topic. It's yet another one of those many rules discussions that I thought had an obvious answer - but clearly, this is not obvious.
And I think that this is the crux of the problem.
Scouring the ruleset to solve the problem is a 3.x style response to an ambiguous corner case. A paradigm shift for DMs and players may need to occur.
The core problem that Hypersmurf described on page 1 is mostly the issue. And it relates to something that Dracorat noted earlier:
Dracorat said:
Actually, it's just that when I apply rules, I apply them consistently. It has come to the point that my players expect that from me. Thus, as a result of any ruling, they will now call me on it if the ruling affects something important, such as this.
I have found in running a lot of 4e, that this just isn't the case anymore. I will tell them that sometimes they will get the +2 bonus from standing on a table and attacking (high ground), but that it MIGHT NOT APPLY later on in a similar situation.
Now, this requires a lot of trust from the players, and this is earned trust by trying to DM the game, not as adversary, but as facilitator. My players know that I am not out to screw them, and that sometimes rulings will bend one way or the other.
It is similar to the "say yes" paradigm (or 4eDMG Pg 42) that bubbles up from editions prior to 3e.
This isn't a "say yes" issue as much as it is a "make a situational ruling and keep on playing", but it's the same idea. DM makes a judgment call, and it's quite possible that it may not be consistently ruled the same way across the life of the campaign.
This is a more narrative than gamist approach, but its how I sense that 4e has progressed. DM fiat isn't always a bad thing. We did it for years in 1e and 2e, without much difficulty, but for players and DMs weaned on 3.x, it can be a bit jarring.
Personally, in my game, I would give the Fighter a perception check to see if he can notice a shift or some kind of movement. Maybe it will provoke an OA or maybe it will be the interrupt from his mark. If someone else can still see the target, I would give the fighter a bonus (to offset the penalty due to invisibility). Make a decision and don't worry about the application of the ruling for the future.
And all of this goes on in the head of the DM. To the player it goes:
DM: Your mark just went invisible.
Fighter: Can I try to hit him if he moves away? I have him marked.
DM: roll a d20
Fighter: Cool! A 17.
DM: OK, you notice that there's a shimmer in the space next to where you thought your mark was. You think he just shifted.
Fighter: [rolls Combat Challenge attack]
Or it can go like this:
DM: Your mark just went invisible.
Fighter: Can I try to hit him if he moves away? I have him marked.
DM: roll a d20
Fighter: Crap! A 10.
Wizard: I see him, he's just moving away to safety.
DM: I'll give you a bonus to your roll...
DM: OK, you notice that there's a shimmer in the space next to where you thought your mark was. You think he just moved away, so roll to hit!.
Fighter: [Rolls melee basic attack].
Just my 2cents...