The drum turns but the igniter doesn't glow, there is no clacking sound as the gas valves open and there is no flame ignition sound...

The igniter was replaced in 9/2007....

I checked the resistance of the coils and they check out ok, and the radiant sensor also checks out ok.

Could the igniter be faulty even though its just a few years old? I checked it for continuity and it tested ok?

what (and how) else to test?

Next test (VERY TRICKY) Remove the 2 wires on the sensor. With the dryer going and set for heat touch those two wires together. The ignitor should light up. Give it time to glow bright and seperate the wires. If the gas comes out that means the flame sensor is bad. Have somebody there or be real close to the wall outlet so you can unpwug dat power fer safety sakes. That is how it is done in the field OR you could just check the sensor for continuity. If the sensor is fine then the ignitor is the next logical thing to check. Do you know for sure you have power to the gas assembly. Unplug the quick connect to the gas assembly so you won't have anything but the gas assembly to test.power it up with a suicide cord. Be careful not to short out the wires. Use small aligator clips. People used to bring them down to the appliance parts shop for me to test. The gas assembly can be easily bench tested. I would connect up the gas and fire up the gas assembly on the bench, touch the wires together bypassing the sensor and then watch fer ignition. I did on a bench with a safe setup. So if you wish not to try that I would not blame you. You have to test the gas assembly 5 or 6 times to see whether or not the coils fail so that is why it is done like that.