Korea, and Seoul in particular, is one of the most wired places on earth. Or should that be most wireless places on earth?
Internet access is very fast (speedtest.net showed 30/10 Mbps down/up at my hostel, about 10 times the speed in North America).
Many cafes, restaurants and bars have free wifi (though Starbucks does not).
There is wi-fi across much of the city, though not free. I didn’t need it enough, but I think it was $2-3/day.
Around town (and especially in metro stations), they have large touch screens with area information, including google satellite map of the area. Very cool.
Surprisingly my GSM phone did roam in Seoul (and Tokyo airport), though I didn’t make calls. (Tip: if you call your phone from Skype in order to check voicemail, turn off your phone first or you may be hit with roaming fees; also, you might need to wait an hour for the phone to be unregistered from the roaming network).
“Seoul Subway” Android app by Sungpil Jang (probably an IPhone version too) – way cool, even offline. Even offline (i.e. no data plan nor wi-fi connection), it will calculate a route between 2 stations including times of the next train(s), and tell you the car and door number for quickest transfer between lines (i.e. where the stairs/hallway to the next metro line is exactly).