I would rule that a person with mage armor could strike an incorporeal being.
Here is why. Mage armor creates a force effect. Incorporeal beings can be affected by force effects whether they originate from spells or some other source.
The thing is, I would not rule that it would be a simply unarmed attack. Mage armor does not in any way inhibit spellcasting. I take that to mean that mage armor is the perfect fitting armor. Essentially it reduces the spell failure chance to 0. Now looking at full plate which has the highest spell failure chance (IDHMBWM - I think it was 55%) as a guideline, and scalemale or a chain shirt which offers similar AC benefits to the spell, essentially the spell is offering you the equivalent of a supermithril version of those armors, if you will. Now also consider the rules for gauntlets - spellcasting is impossible while wearing gauntlets - its not that there is a spell failure chance, but it is flat out impossilbe.
From this above information, I take mage armor to be similar to a suit of armor in that it covers some of you, but not all of you. I would take it to mean that it does not cover your hands as that challenges my sense of consistency between the armor rules and the gauntlet rules. Also, gauntlets are offensive and armor is defensive and the original intent of the spell was as defense, that much is obvious.
However, as I said initially I would still allow strikes against incorporeal opponents from a mage armored individual. The catch is they would have to be strikes with the main portion of the body and not the limbs. Any of you who have ever seen drunken master, or better yet seen a football game, know that this is both possible, and could hurt quite a bit. I would keep the same damage as a normal unarmed strike for that particular would deal, however, I would change the mechanism of attacking. As there must be bodily contact I would probably rule that something similar to a bull rush or grapple had to occur where you would have to enter the opponents space provoking an AoO (and if you had improved grapple, or improved bullrush, I would allow you to bypass that AoO). After entering the opponents space, I would then have you make a regular attack and if successful, you would deal your appropriate damage (provoking another AoO if you did not have IUS). Then, following the model of improved shield bash, I would have you roll opposed strength checks to determine if you had to move back to the space you came from before you entered the opponents square or the opponent had to move back a square relative to the direction you entered his.
I would be most conducive to this sort of attack if someone wanted to charge the incorporeal entity to damage it or bull rush it, simply because it seems to make the most sense to me. Iterive, or multiple attacks really tend to stretch my imagination with this method of attacking (with the body) and so I would probably make this particular attack a standard action rather than an attack so it could only be done once and to encourage the appropriate method of attacking (charging or bullrushing).
So, I would allow it, but it would be difficult - most likely provoking 2 separate AoO (and you can be sure that if my players started doing this as a regular tactic they could be sure they would face a warrior ghost with combat reflexes). I tend to hate to tell my players absolutely NO, especially when something seems to make sense as this does (despite drifting from the designers original intent). I do this mainly because I like to encourage innovativeness and creative thinking in my players (often times because that is the only way to overcome obstacles I create).
