Korean Pop is big business in Japan - you see and hear a lot of it on TV and while walking out n about town. K-POP has been around for a while in Japan but seems to have really taken off last year in 2010 with groups like Girls Generation (少女時代) who reached top 2 in the Oricon charts with "Gee" and KARA reaching top 5 with "Mister."
Girls Generation (also known as SNSD) and the Japanese version of their tune "Genie" below.
And the Korean version below.
Go into a book shop and you are likely to see Korean singers on the cover of many mags too. Here we have more Girls Generation. Their "Gee" is just so awesome - Korean version below.
And the Japanese version below. I saw that some Japanese folks complained that they localized the lyrics for the Japanese market and preferred the original Korean.
Video of the girls performing at Odaiba.