If you're hosting on your iDisk I don't think it's possible without using some third party service that stores the comments on their servers. HaloScan is such a service that springs to mind, never used it myself tho'
I don't have the time to find the thread here for you but do a search on iWeb. There is a free 3rd party solution. Maybe you can find it by doing a search on iWeb at versiontracker.com