If you unlocked it with HTCdev.com, then yes they will be able to. But usually unlocking the bootloader doesn't make the entire device stop working. They might blame you for over clocking or something, but there isn't anything you can do about it. You obviously can't re-lock it, so I would just send it in and hope that they don't care.
Well, it depends. Usually, HTC will only check for the TAMPERED flag and the bootloader status. But sometimes, they may have a problem with the phone being S-OFF.
I S-ON'ed my phone before giving it for warranty repair.
Anyways, whether u will S-ON the phone or not, ill tell u how to S-ON.
Put the phone in fastboot mode. On ur PC, open command prompt and type:
fastboot oem writesecureflag 3
HOWEVER, DO NOT S-ON UR PHONE IF HBOOT IS 1.55 OR ABOVE. THE TAMPERED LOGO WILL RE-APPEAR