Depending on the device, unlocking can be permanent or temporary. Using the HTC dev tool to unlock is indeed permanent and does in fact void the warranty by recording your device's serial number in their database of unlocked devices.
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Current Devices: Nexus 5, HP 14 Chromebook (Data Enabled), Google Glass, (2013) LTE Nexus 7, Chromecast
Retired: Samsung Fascinate, Droid Charge, Galaxy Nexus, (2012) WiFi Nexus 7, Asus TF700, Nexus 4, Logitech Revue