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Reverse records are PTR records, not A records. That doesn't matter, though, because PTR records have to be served out a different domain anyway. Got the linode manager (not DNS manager), select the network tab, and you'll find a box for setting the reverse DNS entry.

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The irony is that Bill Gates claims to be making a stable operating system and Linus Torvalds claims to be trying to take over the world.
-- seen on the net

Are you trying to set the reverse dns to "thoughthole.com" or something else? It will only let you set the reverse dns entry to something that resolves to your linode's IP address.
It might just be a matter of patience. It can take up to a day or two for the changes you made to your forward dns to completely propagate through everyones caches. Try again tomorrow