Let's assume that you have a task sequence called "Windows 7 Enterprise - Install", and you have deployed that to the "All Unknown Computers" collection. In this scenario, when you PXE boot a never before deployed system (that's what you refer to as bare-metal)
the system will be treated as Unknown to ConfigMgr (assuming that you've enabled Unknown Computer support on the Distribution Point). From what you've described earlier, this is already in place in your environment and that's how you do OSD.
Now to the re-image part (Refresh scenario). The easiest way to refresh a machine is to just re-install the operating system completely, do a total wipe on the drive and begin from scratch (keep in mind that this is absolutely not the best way to do it).
Since your system (a client computer in this case) has already been deployed before, ConfigMgr already knows about it and therefor the Unknown Computer support will not pick it up.
What you need to do in that case, to enable the system to be refreshed, is to follow the instructions below:
1. Create a new collection, call it "OSD - Windows 7 Enterprise - Refresh".
2. Add the client computer (a workstation, laptop etc) as a Direct membership to this collection.
3. Locate your task sequence called "Windows 7 Enterprise - Install" and deploy that to your newly created collection called "OSD - Windows 7 Enterprise - Refresh".
4. Now PXE boot the client that is supposed to be refreshed, and you should be able to once again apply the OS of your choice.
Keep in mind though, that refreshing a computer involves many tricky steps. What I described here now is only the very basic way to accomplish this. I hope it gave you a bit more insight.
Regards,