anon said:
So the question of 1 aura (me) or 2 auras (me + hat) is unclear?
No, not in my opinion. The rules are pretty clear about what detect magic does/does not detect. If the DM decides to do as Maleketh suggests, he has opted to house rule it (and DM's can do that, but the RAW are pretty clear).
Round 1 - He detects the presence of magical aura/auras in your direction
Round 2 - He detects the number of magical auras in your direction. He also detects the power of the most potent aura.
Round 3 - He detects the strength and specific location of each aura.
In my game, I have always indicated the aura's as a faint glowing surrounding the item/object/person. If there are multiple auras, they are differentiated by color/flavor text. The more powerful, the more "glowing" the aura is.
So, specific to your scenario, it would proceed as follows:
Round 1 - The presence of magical auras in your direction is detected
Round 2 - He detects 2 magical auras, one of which detects as faint magic (both are actually faint, but he has no way of knowing this at this point).
Round 3 - A faint aura is detected on/about your head (it would actually detect specifically to the hat, in whatever form it currently is). If spellcraft is made, it is shown to be from the illusion school. This is from the hat. A faint aura is also detected over your entire body, indicating a spell effect is on you. This is the disguise self function of the hat. It would detect, if spellcraft is made, as illusion as well.
So two distinct auras, both faint and assuming that spellcraft is made, both from the illusion school.
Make sense?
Sources:
http://www.d20srd.org/srd/spells/detectMagic.htm
http://www.d20srd.org/srd/magicItems/wondrousItems.htm#hatofDisguise
http://www.d20srd.org/srd/spells/disguiseSelf.htm