Not just for 2.1, but when making any kind of upgrade of WP, a theme, a plugin, etc., it's best to have a local copy of your WP site on which you can test the changes as much as possible before permanently uploading them to your actual site. Basically you have a copy of your site with all the plugins, theme changes, etc. on which you PRACTICE and TEST as many times as necessary until it's ready to be uploaded to your live site. This will greatly minimize unexpected issues that severely affect your site in real time.
There's an excellent tutorial of how to do this using a USB thumb drive at http://www.tamba2.org.uk/wordpress/usb/. The advantage of this approach is you can run it from any computer the thumb drive is plugged into. (There are other approaches as well, such as if you search the forums for "Xammp".) This actually loads not just WP but the Apache web server, MySQL, and everything you have to deal with for your regular site.
Test changes here and THEN upload them to your real site, even when it's just a single plug-in that the author's updated. (Or to work out the kinks in a new theme.) It's not 100% (not many things in life are), but it's far more likely to drastically reduce the possibility of your actual site going down or its functionality reduced.
I would like to also take this opportunity to suggest that this is why everyone should vote for the idea suggested at http://wordpress.org/extend/ideas/topic.php?id=70 that future versions of WP have this capability built into it, where you can test changes before migrating them to the live site. The issues reported so far concerning 2.1, IMHO, make this something we seriously need.
Hope this friendly advice helps.