Backgrounds jobs are expensive to setup and are not reusable. PowerShell MVP Oisin Grehan has a good example of PowerShell multi-threading.
(10/25/2010 site is down, but accessible via the Web Archive).
I'e used adapted Oisin script for use in a data loading routine here:
http://rsdd.codeplex.com/SourceControl/changeset/view/a6cd657ea2be#Invoke-RSDDThreaded.ps1