After testing this, I am going to say no. My test was run on Windows Vista with PowerShell V2 CTP 2. I first created a global variable in one shell.
$Global:Var123 = “Hello World”
I then verified it by typing Get-Variable.
I opened a second PowerShell shell and typed $Var123....nothing. I then executed Get-Variableand confirmed that the variable was not present in the second shell.