Amazon explicitly allows you to add DRM-free MOBI ebooks to your Kindle, so this can't void your warranty.
The Kindle 3
License Agreement and Terms of Use says:
Quote:
1. Digital Content
Use of Digital Content. Upon your download of Digital Content and payment of any applicable fees (including applicable taxes), the Content Provider grants you a non-exclusive right to view, use, and display such Digital Content an unlimited number of times, solely on the Kindle or a Reading Application or as otherwise permitted as part of the Service, solely on the number of Kindles or Other Devices specified in the Kindle Store, and solely for your personal, non-commercial use. Unless otherwise specified, Digital Content is licensed, not sold, to you by the Content Provider. The Content Provider may post additional terms for Digital Content in the Kindle Store. Those terms will also apply, but this Agreement will govern in the event of a conflict. Some Digital Content, such as Periodicals, may not be available to you through Reading Applications.
Limitations. Unless specifically indicated otherwise, you may not sell, rent, lease, distribute, broadcast, sublicense, or otherwise assign any rights to the Digital Content or any portion of it to any third party, and you may not remove or modify any proprietary notices or labels on the Digital Content. In addition, you may not bypass, modify, defeat, or circumvent security features that protect the Digital Content.
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This only mentions Kindle store ebooks, so it would not violate these terms to strip the DRM from a MobiPocket ebook, or an ePub, and then download a DRM-free MOBI version to your Kindle.
It does violate these terms if you strip DRM from an AZW or TOPAZ ebook you bought in the Kindle Store and then download it to your Kindle. However, I have never heard of any adverse consequences of doing so.