[douglas]

Oh, you can wear any kind of armor you want and get the full AC bonus, but if you're not proficient with it you get its armor check penalty applied to all of your attack rolls and strength and dexterity based ability and skill checks. For half-plate, that's -7. If you were a wizard rather than a psion, you would also suffer from the armor's spell failure chance. So, if you don't plan on doing much attacking (that includes psionic powers that require touch or ranged touch attacks) or exerting yourself physically, go right ahead.