I seriously doubt Google/HTC would really be able to deny a warranty claim for a bootloader-unlocked N1, at least for customers in the US. See the Magnuson-Moss Warranty Act. A manufacturer can't deny warranty coverage based on a user modification that has no bearing on the defective part.
If the trackball falls out of my phone and I send it in for warranty repair, there's zero chance they'll be able to successfully claim that the problem was caused by me unlocking the bootloader.
If the trackball falls out of my phone and I send it in for warranty repair, there's zero chance they'll be able to successfully claim that the problem was caused by me unlocking the bootloader.