Yes, you can boot from a hard disk in the Ultrabay. However, it is not a 'snap' to select the boot disk. The most modern ThinkPad I have used is X41T, and on that computer, it was
a snap. X41T has a newer generation of firmware, which provided a full list of bootable devices on the F12/"Select Boot Device" menu. As I understand it, any ThinkPad that supports Predesktop Area
allows for really easy boot device switching. The boot device selection list on earlier ThinkPads (including T-twenties) is much more limited: it has only four options (network, removable device, optical, hard disk). If you have more than one of each type, then there is no quick way to choose which to boot from. To specify which hard disk to boot from, you will need to change the order in BIOS. It will be a real inconvenience if you intend to alternate between Windows and Linux often. Entering BIOS, changing boot order, and saving changes then rebooting will add a minute to your boot time.
I believe GRUB is fairly advanced and can even chain-load other bootloaders, on other drives. If that is possible, then what you could do is configure GRUB, as an option on its list, to boot from another disk. Configuring GRUB is beyond my abilities, but I am pretty sure that is possible.