MEM Controller issued an IFR clearance to an aircraft that was below the MVA at the time of issuance; resulting in an operational error.

Narrative

I was working MEM Departure Radar; when a Cherokee (PA28); who had just departed a satellite airport; called to pick up a pre-filed IFR clearance. Once I had issued a beacon code and radar-identified the aircraft; I discovered he was at 1600MSL; but was in a 2500FT MVA (minimum vectoring altitude) area; and was headed toward the obstructions for which the MVA was created. Also; there were thunderstorms north of MEM; and I was unsure of the pilot's flight conditions. So; I issued the IFR clearance and a turn northeast-bound; away from the antennas and the MEM departure corridor. Later; the scenario was reviewed; and I was informed I'd been involved in an operational error. Apparently; I should have either: A. asked the pilot if he could maintain VFR until 2500 (unknown if this was possible) or B. not issued a heading until the aircraft left 2500; but that would have allowed him to possibly stray too close to the MEM departure corridor. In this case; I feel I followed the intent; but maybe not the letter; of the law.