If I want to put a Wordpress (3.2.1) site into Maintenance mode, is it still necessary to use a plugin? If so, is this a good option?
Yes there is. But it is primarily meant for short interruptions (such as during upgrades) and not very user-friendly. WP creates
.maintenance file in root (with timestamp info about time) to trigger it and removes after it is done.
See WordPress Maintenance Mode Without a Plugin for reference.
In practice using a plugin is usually more convenient and functional.
There is no maintenance mode in core AFAIK. And as far as plugin's go, I prefer JF3 Maintenance Redirect as it allows IP address whitelisting.