I'll add to this thread since it already has good info.

My bike was air-freighted to Incheon. Before I could get it released from customs I needed the aforementioned temporary tags and insurance. I was able to get both at the governmental district office, three subway connections from the airport train station. The subway system is very easy to navigate, with announcements and station signs in Korean and English. (I was originally sent to Incheon City Hall, which does not handle this type of matter.)

My customs clearance agent informed me that I would need to post a cash deposit of about US$2000 (on a DR650), but he was able to have them use the carnet instead. In so many words he intimated that the CAA-issued carnet was usually not accepted, but the customs clerk was new and didn't quite know that.

I was able to uncrate at the customs warehouse (as entertainment for anyone taking a smoke break) and dispose of the metal-frame crate alongside the building for no cost.

There is another way to get from from the island of Incheon to the mainland if you have trouble using the expressways. There is a car/passenger ferry that can be taken from the docks at the east end of the island. From customs, go to the main airport highway and follow the signs for Yeong Jong dock. Cost was about US$6 (motorcycle), trip was about 20 minutes. You are let off at Wolmido docks on the mainland.

This is from a holder of a US passport, on a US-registered bike, in August 2010.

Cheers
Sean