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If you do some searching, you'll find a procedure on how to fix this yourself. There are three or four tiny screws located under the rubber focus ring. Tightening those screws applies more pressure to some plastic pads, so the lens will be less likely to creep.
You could send it to Canon while under the warranty, but you'll likely have to make the adjustment again in the future.
I've tightened the screws on mine a couple of times. Now I don't bother anymore.
I've read about this theory many times... but has it actually resolved the problem for anyone? I tried it, and it did not make a difference.
... who looked again right after posting this to see that the prior posted did get a fix out of it! I guess I'll have to try it again! :-) Mine is also in that category of "doesn't creep in normal use but may when pointed straight now."