If you're looking to get FLVs to playback in Quicktime I believe your best route is Perian.  It installs a preference pane in your System Preferences.  It's simple to install and update.  Another great thing about installing Perian is you can convert FLVs to other formats using Quicktime Pro.

Also if you want to quickly preview FLVs from within Finder using the Quick Look feature try using this Quick Look plugin.  Place the flv.qlgenerator file in your Library/QuickLook folder.  This can go in your root library folder or your user library folder.

Hope this is helpful, I use it everyday.