This is true for not just DBAs, but everyone who does anything that is not plain "a trained monkey could do it" work. Make yourself irreplaceable in other ways, but not with your everyday processes.

Our lead programmer at my first job always told us, "Document everything. If you get hit by a beer truck on the way home from work tonight, someone needs to be able to pick up where you left off." Especially if you're leaving for one of the less morbid reasons, you don't want to burn your bridges with your co-workers when you might want to use them as references down the line.

The "replaceable = able to take vacations" is right on the mark too. My entire team has been struggling to get our (constantly multiplying) "special case" processes documented and get others cross-trained so we don't have to be tied to our cellphones (or, worse, our computers and an acceptably secure net connection) when we take vacations. It doesn't always work, but it's gotten to the point where most of us only get one or two calls from our backups over a week's vacation as opposed to several-times-daily ones. Makes for a much happier team, and our users/clients are happier as well since they don't have to wait as long for resolution/completion when the primary is out.