Yes the metbabase is machine specific as it contains SIDs and machine specific information. However there is a tool to allow you to export metabase keys etc to other machines. Its called iiscnfg. Just google iiscnfg for links to all the info on the MS sites.
With that tool you can export all IIS/Metabase config for a single site (or more) and import them into a a different server.
You could even use it to script scheduled backups of a single sites metabase data to a text file. Then use that file to re-import when required. Its very handy for such things.
Its a great tool and we have used it many times for IIS server migrations and even server merges.
Only things to watch out for are complex things like SSL certificates (manual pfx export/import required, Frontpage Extensions etc.)
There are a few issues with iiscnfg concerning security. But there is a great link below with modified versions that overcome those issues:
http://aspalliance.com/1038_IISCnfgvbs__IIS_Settings_Replication
Hope that helps with your sites config backup plans.