It can be customised by changing the file linux_logo.h in include/linux directory. its a c header and pretty hard to change by hand however there is a plugin available for gimp
http://registry.gimp.org/detailview.phtml?plugin=Linux+Logothat will create one for you. all you need is a picture 80x80 with less than 224 colours. you can either let the plug in create the 3 varieties (2,16,224) or create them yourself and use them with the plug-in. it will ask you where you want to store the file and if you are game you can put it in ($SRCDIR)/include/linux/linux_logo.h. once that is finished all you need to do is recompile the kernel as usual, reboot, and if framebuffer is working you will see your new logo upon bootup.