They will definitely support Symbian for a while. For the most part, Symbian^3 appears good enough, but S60 V5 at this point should be cast away.
At this point however, there is nothing on Symbian^3 that will draw back users that have moved out to iPhone, Android or even Blackberry, but Nokia can still catch users migrating out of featurephones into smartphones for the first time and probably able to keep some Nokia users migrating out with the new devices being introduced. Also at this point, Nokia is suffering from brand erosion, especially on South East Asia where it has traditional stronghold.
The four new devices (C6-01, C7, N8 and E7) appears it can hold the fort for a while, maybe give a vital short term life injection on Nokia, but they need to follow up with even more devices to keep the "buzz" going.