I've got a site of about 500 pages from a client, all attached to a template. Is there any way I can do a "bulk detach" rather than going through each file individually?
I assume that you're working within Dreamweaver. If you have the site that is based upon a template displayed within the Files panel in Dreamweaver, go to: Modify > Templates > Export without Markup...
You'll be asked where you want to save your files to. If you plan on doing this only once, then uncheck the two checkboxes on the UI. If you need to do it multiple times, look at the help files they may be able to let you know when you might or might not want to select those options.
If you want to stop working with the "old" file and keep working with the files within Dreamweaver, I'd export the files without markup to another folder, then change the site definition to point to the new location, or perhaps swap the contents of the folders with Dreamweaver closed (to prevent it wanting to look through everything to try to find updated links)
Note: At this time, there's a bug in Dreamweaver (I've seen it in CS3 and CS4) that was introduced by Microsoft in Vista SP2 and Windows 7, the browse for folder dialog may select a folder above the one you think you're selecting see: http://forums.adobe.com/message/2183285 ). So you'll either need to dig down one further folder, or make sure you verify/update the folder path after you've selected a folder in the browse folder.
If you're not within Dreamweaver, then you'll need to do a good bit of find and replace.