Comment 5 for bug 508173

You can almost certainly work around this by preseeding /dev/sda3 rather than (hd0,3), and in fact I strongly recommend making this change to your preseed file; we are in the process of deprecating the (hdX,Y) names for most purposes other than manual exploration at the GRUB rescue prompt.

Nevertheless, this is still a bug in how we're handling preseeding following recent changes.