Seems there's a lot of misinformation here, as it does not seem like a 3rd party engraving voids the warranty.
Will the custom laser engraving on my iPad be preserved?
If your original iPad was personalized by Apple and you arrange service by calling Apple Technical Support, you will be asked if you want your replacement iPad to be personalized with the same text. iPad personalization is not available in all countries or through Express Replacement Service.
If your original iPad was custom laser engraved by another company, your replacement iPad will not be personalized.
This is from here
http://support.apple.com/kb/index?page=servicefaq&geo=United_States&product=ipad