This thread on activerain suggests that an appraisal won’t be required, at least by HUD. I’d imagine the lender could set their own requirements:

http://activerain.com/blogsview/1195842/fha-loan-assumption-process-how-it-works-

And now for the obligatory “moral hazard” issue … According to HUD it’s possible to assume an FHA mortgage, default, then request a loan modification almost immediately:

http://portal.hud.gov/hudportal/HUD?src=/program_offices/housing/sfh/nsc/faqassum