Is answering a bounty question immune to the daily reputation limit?
Yes and no.
It depends on "when". If you get the bounty after you've maxed, then yes; it is immune.
But if you get awarded a bounty at 2am (server time), then yes, you get the bounty, but you'll simply "max" sooner.
It is (in some ways) very frustrating to get for example a 150 bounty first thing in the morning (server-time). But hey, you still get the bounty!
Marc's answer used to be right, but as of March 2010, it is no longer correct. Bounties are now completely, totally, 100% immune to the rep cap. See the blog for the full story.