Messages on bomb
From Halopedia, the Halo wiki
The best way to see it is to perform the No weapon glitch and pick up the bomb, walk to a wall and hold R so the bomb is quickly bouncing between you and the wall. If this does not work because the wall is too light/the bomb is too much to one side of the screen, then look directly up and hold R and you can see it as it's thrown into the air. Also, it helps to use a Spartan player, as opposed to an Elite, because the dropping animation is different. This difference means that the bomb drops directly down, not forward first.
An alternative method is to simply grab the bomb and to drop it into a shiny area of the map, and then zoom to focus at the bottom of the bomb with a Sniper Rifle/Type-50 particle beam rifle. The messages are much more visible in these conditions. Certain matchmaking modes based on community maps in Halo: Reach (particularly in Grifball) also show a close-up of the bomb during Loadout screens.
Halo 2 Messages
The messages seem to correspond to what map is used:
'Thus I Refute Thee':
'Hold Onto Yer Butt':
'Kiss It Goodbye':
A hieroglyph of a Spartan helmet, next to the two fingered peace symbol followed by "xoxo ♥":
Halo 3 Messages
You will see a different message, whether or not you are the Connection Host. If you are the connection host, the message seen is "Hold Onto Yer Butt." If you are not the connection host, the message seen will be "Thus I Refute Thee."
Halo: Reach Messages
In Halo: Reach, the messages return unchanged.