Will using a refilled cartridge void my printer warranty?
US Federal law prohibits any company from requiring you to purchase only their products and prohibits a manufacturer from voiding your machine’s warranty simply by using alternative cartridge solutions. See Magnuson-Moss Warranty Improvement Act below for more details.
The law excerpt below indicates that a printer manufacturer cannot void the printer warranty due to refilled cartridges, unless they can prove that the refills caused or contributed to the failure in the printer.  Under no circumstances may any company or associate, salesperson, or service technician deny you service or warranty work that would otherwise be covered under a warranty program simply because the cartridge was refilled.  The U.S. government protects your consumer rights.
If a representative threatens to void the machine warranty or charge for a service call because you are using a refilled ink supply, ask for their statement in writing.
United States Code Annotated
Title 15 – Commerce and Trade
Chapter 50 – Consumer Product Warranties
15 Section 2302
“(c) No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer’s using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade or corporate name; except that the prohibition of this subsection be waived by the commission if:
1) The warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and
2) the Commission finds that such a waiver is in the public interest.”
Here are some interesting links:
This doesn't just apply to ink and toner. This can be applied to other products bought.