I haven't had any problems cloning to an an 8GB memory card with 7.5GB VFAT and 0.5GB ext2 partitions (either on OS2007 or OS2008, on N800 and N810).

So 8GB SD cards definately can be booted from, I'm not sure why some of you are having problems though - the obvious answer would be that the partitioning has gone awry.