Here are some powershell functions (using XPath) that come in handy when working with SSIS packages:

function FindConnectionManagerNames {
	param($fileName)
	$xml = 1 (Get-Content $fileName);
	$ns = New-Object Xml.XmlNamespaceManager $xml.NameTable;
	$ns.AddNamespace("DTS", "www.microsoft.com/SqlServer/Dts");
	$xml.SelectNodes("//DTS:ConnectionManager/DTS:Property[@DTS:Name='ObjectName']", $ns) | Foreach { $_."#text"; }
}

function GetConnectionManagerConnectionString {
	param($fileName, $connectionManagerName)
	$xml = 1 (Get-Content $fileName);
	$ns = New-Object Xml.XmlNamespaceManager $xml.NameTable;
	$ns.AddNamespace("DTS", "www.microsoft.com/SqlServer/Dts");
	$xml.SelectSingleNode("//DTS:ConnectionManager[DTS:Property='$connectionManagerName']/DTS:ObjectData/DTS:ConnectionManager/DTS:Property[@DTS:Name='ConnectionString']", $ns)."#text";
}

function FindVariables {
	param($fileName)
	$xml = 1 (Get-Content $fileName);
	$ns = New-Object Xml.XmlNamespaceManager $xml.NameTable;
	$ns.AddNamespace("DTS", "www.microsoft.com/SqlServer/Dts");
	$xml.SelectNodes("//DTS:Variable/DTS:Property[@DTS:Name='ObjectName']", $ns) | Foreach { $_."#text"; }
}

function GetVariable {
	param($fileName, $variableName)
	$xml = 1 (Get-Content $fileName);
	$ns = New-Object Xml.XmlNamespaceManager $xml.NameTable;
	$ns.AddNamespace("DTS", "www.microsoft.com/SqlServer/Dts");
	$xml.SelectSingleNode("//DTS:Variable[DTS:Property='$variableName']/DTS:VariableValue", $ns)."#text";
}