Hi, i had also the issue with proximity sensor, tested by i.e. sensordump app shown no activity at all so I thoght it is broken. I sent it to HTC service centre for repair. They did just recalibration of the sensor and all is working fine now.
Seems there is no any tool nor any hidden service menu on HTC Desire like i.e on ZTE devs that enables you to recalibrate it by yourself :(
Sent from my HTC Desire S using XDA App
HTC Desire S
S-OFF (by Revolutionary)
Recovery: 4EXT Recovery Touch v18.104.22.168 RC 3
Rom: Fallout v5.0 with Sense 4.0A (thx lowveld)
Kernel: 3.0.42 (thx amidabuddha)