Well... this fixed it. But now I'd like to know if anybody knows WHY it fixed it...
#this fails
$from = "\\srvrname\r$\SQL\MSSQL10_50.instname\MSSQL\Backup\srvrname$instname\DBNAME\FULL\srvrname$instname_DBNAME_FULL_" + $dd + "*.BAK"
#but this works
$from = "\\srvrname\r$\SQL\MSSQL10_50.instname\MSSQL\Backup\srvrname" + "$" + "instname\DBNAME\FULL\srvrname" + "$" + "instname_DBNAME_FULL_" + $dd + "*.BAK"