Every few upgrades of Ubuntu I seem to get into a situation that I want or have to do a clean install, then waste time getting the new system back into a workable state. It would have to be pretty lightweight and intuitive for my purposes.
Github has a project called Boxen that does this for Mac OSX: http://boxen.github.com/
Google uses their own Puppet solution for their developer workstations: http://www.youtube.com/watch?v=YlKXRdSAZhY
Ansible will work great for this too - particularly if you're on Linux.
* Keep your dotfiles somewhere that can be synced back easily (e.g. Dropbox, VPS with rsync, git repo, whatever)
* Make all dotfiles symlinks to the above. This way, if anything were to blow your hard drive up, it will have a relatively painless recovery.
* Keep workspace stuff/code/things not too easily replaceable in a separate physical drive if possible. Mount this drive to whatever directory that would make sense to you through your fstab or whatever method you prefer. Adding the nofail option in your fstab is also a good idea if you won't have it connected all the time. Reason for all this: You can now wipe your Linux HDD more confidently if you ever need to do so. Important dotfiles and configs should all be safe, code is on a separate drive and other stuff is backed up off-site hopefully.
You have to be wary when apt-get is only one command away but in general it works quite well - I'm not editing so many global configs and got most of them in my dotfiles repo anyway.