IOS55 is an IOS that newer games appear to use -- "Need for Speed: Undercover", "Sonic and the Black Knight", "Wii Sports Resort" and "The Conduit".
This IOS is presumably required by Undercover to make a Logitech USB steering wheel work. However, this does not explain why the other games require it although, in the case of The Conduit, it is possible that there is a connection between its usage and support for the Wii Speak peripheral (it has been reported that both IOS38 and IOS55 are required by the official Nintendo Wii Speak channel).
Typically, if a game that requires this IOS is started without it (via brickblocker or a Starfall update skip, for example), then the screen will turn blue and a white-text error message "Error #002" will appear. The Conduit behaves somewhat differently as it will simply display a black screen before freezing.
Donkey Kong Jungle Beat (Wii) and Excite Bots appear to require IOS55 as well.