Join Date: Mar 2011
Location: Heavily Spanking Saint Celestine
Might wanna check p. 500 of the rulebook, the section on 'Magical Interference'.
Yes, the Dragon Mage can cast spells whilst wearing Dragon Armour (since it is listed in his specific entry), and is even allowed to purchase an (additional) item of magical armour from the magic items. I'd even recommend that ... a Charmed Shield goes a long way on a character riding a monster mount.
40k Armies: Dark Eldar / Adepta Sororitas
WHFB Armies: Vampire Counts / Empire / various Elves
Horus Heresy: Alpha Legion / Warp Cult