This MacRumors report seems to border on legal advice:
"Apple remains free ... to discourage users [from jailbreaking] by ... voiding product warrantees due to violations of the terms and conditions all users must agree to... ."
This may just mean that Apple is free to TRY to discourage people, but to me, this implies that the writer feels Apple is within its legal rights to dishonour warranties based on jailbreaking.
If the contract that consumers make with Apple by agreeing to the terms and conditions is considered by the law to supersede any other rights the consumer has, this may be true.
However, that contract may NOT supersede consumers' rights, depending on where they are. I would urge people to read any relevant consumer protection legislation in their jurisdiction (some such laws, at least, use reasonably plain language and aren't that hard to get around). In BC, for example, the Sale of Goods Act (available at
http://canlii.org/) imposes an "implied condition that goods will be durable for a reasonable period of time having regard to the use to which they would normally be put and to all the surrounding circumstances of the sale or lease" (read the Act for the full wording; it's section 18(c)). Also, in a retail sale, that implied condition cannot be gotten around by any warranty or condition imposed by the seller (s. 20(2); again, read the section, as there are some exceptions).
I don't know of any particular examples of courts considering what is "reasonable" etc. but we cannot conclude for certain that a retailer can arbitrarily deny repair of a phone just because of a breach of the manufacturer's terms and conditions--consumer laws may forbid this in some circumstances.
One wrinkle though is the legislation cited above gives consumers rights against the seller, not the manufacturer. If you buy directly from Apple, then presumably your beef is with Apple. If you buy from someone else (like your phone company) then your rights may be enforceable directly against them.
This isn't legal advice, of course, and if you have a question about your rights you should do your own research and consult a lawyer that practices in your jurisdiction.
What you should NOT do is assume that you have no right to question any retailer's denial of warranty, especially, for example, if your complaint is about a defect unrelated to any jailbreaking of the phone.