My blog was never validating when using WordPress Mobile Pack, because an ambersand contained in a link in the barcode widget. I had some time to find in which file it should be repaired, and I had to do it again everytime I was upgrading. If someone needs to (and dear developer please include the patch so we don't have to do it every time) edit the file wpmp_barcode.php in the directory
Change line 93 from:
$link .= "&wpmp_switcher=mobile";
$link .= "& amp; wpmp_switcher=mobile";
Note: the parser in this forum is changing the & a m p ; to &. There should be no spaces in the above code.