Saying "radar contact" does not make an aircraft IFR. What if you depart VFR and contact approach at 2500 with an IFR aircraft 500 above you, should the controller not radar identify you? They will radar ID you and if you're on your IFR code, tell you to maintain VFR. When they have approved separation, they will then issue you your IFR clearance. What you got on the ground was your route or expected clearance. If you were told to hold for release or not given a release then you are VFR (uncontrolled airport, takeoff clearance at a towered airport is automatic IFR when you have received your clearance on the ground).
From the 7110.65:
"Treat an aircraft planning VFR for the initial part of flight and IFR for the latter part as a VFR departure. Issue a clearance to this aircraft when it requests IFR clearance approaching the fix where it proposes to start IFR operations. The phraseology CLEARED TO (destination) AIRPORT AS FILED may be used with abbreviated departure clearance procedures."
"If clearance is to destination airport, the phraseology CLEARED TO (destination) AIRPORT must be used."
So, you get your "clearance" on the ground with the route, altitudes, freq, code, etc but not an IFR release and depart VFR, you are VFR. The controller Radar ID's you, you are VFR. When you request your clearance and the controller can legally issue it to you (traffic/mva/etc), the controller then will say, "N12345 cleared to Nashville airport, as filed...direct holly, direct molly, then as filed... etc climb and maintain 5000 expect 7000 in 10." Then and only then are you IFR.
Just my $0.02...If I'm wrong, I'm sure I'll hear about it.