In practice, processes typically go into D state ("uninterruptible sleep") when they're blocked on access to a local disk, whether that's explicit I/O (read/write) or implicit (paging). Not coincidentally, this is also the one type of blocking I/O that you can't get knocked out of by a signal.
Actually, they'll get blocked on access to NFS or other network-based disk, too. (but you can configure the NFS mount to allow signals to interrupt) I've seen NFS problems lead to loads over 100.
