This cannot be generally answered. It depends on what changes have been made when rooted.
If you just rooted the device and installed some apps, but did not modify font, preload or dictionary partition, you should be able to unroot without trace via restoration of the system partition, boot loader, recovery partition and a factory reset.
If the device is obviously rooted, Sony will deny guaranty. If there are some hidden traces, they have to examine the device thoroughly, which I doubt they'll do.
Be sure to check out the Wiki for Sony PRS-T1 Rooting and Tweaks!
Many questions are answered there already... If not - feel free to add
Some modifications will void warranty and may irreversibly destroy the device. I won't take responsibility if you damage your reader by using one of my files or following my suggestions.