Upgraded to PowerShell 3.0 and found a piece in one of our scripts that wasn't compatible. Here is a sample with the corrected code which works in both 2.0 and 3.0. Conclusion: Do not use $MyInvocation.BoundParameters. However, $PSBoundParameters will work on both v2 and v3.

testargs.ps1

param(
[string] $foo,
[int] $bar
)
$arguments = ""
# Change to $PSBoundParameters..
$MyInvocation.BoundParameters.Keys | %{ 
  Write-Host "key $_"
	# $MyInvocation.BoundParameters.Item( string key ) fails in PowerShell 3.0:
	# $paramValue = $MyInvocation.BoundParameters.Item( $_ )
	#
	# Exception getting "Item": "The given key was not present in the dictionary."
	# At C:\temp\testargs.ps1:8 char:2
	# +     $paramValue = $MyInvocation.BoundParameters.Item( $_ )
	# +     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    # + CategoryInfo          : NotSpecified: (:) [], GetValueInvocationException
    # + FullyQualifiedErrorId : ExceptionWhenGetting
	
	# This works in both PowerShell 2.0 and 3.0
	# Conclusion: Do not use $MyInvocation.BoundParameters - it will have the keys but not the values?
	# $PSBoundParameters has both keys and values on v2 and v3.
	$paramValue = $PSBoundParameters.Item( $_ ) 
	if( $paramValue -match "\s" ) {
		$paramValue = """$paramValue"""
	}
	$arguments += " -$_ $paramValue" 
}

Write-Host $arguments