I think Lee Holmes started off something called "BgShell" HERE.
Since then, it has been added as a CodePlex project HERE.
Quote from the site:
"BGShell is a lightweight PowerShell 2.0 host written using Windows Forms."
Features:
* Tab expansion
* Syntax highlighting
* Visual Studio debugger-like tooltips
* Tabbed interface
The last 2 features listed above likely only work on Vista though.
