I can confirm that upgrading from a fresh wubi install of 10.04.1 to 10.10 currently fails to boot after successful completion, both for 32bit and 64bit installs. Only in the 32bit case do I get a grub prompt - in the 64bit case, the machine just reboots after selecting ubuntu from the list of operating systems.
I can also confirm that this behaviour is fairly recent, and that a previous upgrade from 10.04.1 to 10.10 worked in a 64bit wubi environment. In that case, an automatic security update of 10.10 made it impossible to boot.
I can confirm that upgrading from a fresh wubi install of 10.04.1 to 10.10 currently fails to boot after successful completion, both for 32bit and 64bit installs. Only in the 32bit case do I get a grub prompt - in the 64bit case, the machine just reboots after selecting ubuntu from the list of operating systems.
I can also confirm that this behaviour is fairly recent, and that a previous upgrade from 10.04.1 to 10.10 worked in a 64bit wubi environment. In that case, an automatic security update of 10.10 made it impossible to boot.