If passed in its current form, motorcycles larger than 280 cc and newer than model year 2000 would be required to go through annual smog checks beginning in 2012. While cars and trucks have eliminated more than 99 percent of noxious emissions (hydrocarbons, NOx, particulates, etc) since the 1970s, motorcycles are still estimated to produce more than 15 times the emissions per mile.
While bikes have gotten cleaner in as-delivered condition from the factory, many riders have modified their bikes. The annual checks are meant to ensure that even if they are modified, bikes would still need to pass the regulations in effect when they were built.
[Source: Green Car Congress]