According to
this and
this X JAPAN might be preforming in Seoul in August, possibly at the Olympic stadium. Both articles quote YESCOM, the arranger of the concerts, that says that X JAPAN will indeed come there, though there no date has been set so far.
Both articles mention a possible official announcement in May.
The first article also mentions the possibility of L'Arc~En~Ciel going on a big tour - China, Taiwan, France, South Korea, Hong Kong, Tokyo and Osaka.
EDIT 31.05.08: According to
http://www.xjapan-weekend.net/, Yoshiki will travel to Korea and announce the long awaited X JAPAN Korean live in the upcoming week. Note that this is still a rumour and nothing official is yet known. Thanks to
http://theothereast.com/.