It's only been a couple weeks since the Helio announced that all their services will be disconnected in May, yet we've already had a flood of Helions switching over to AT&T to continuing using Korean language features, i.e. texting, emails, web browsing. Here's a quick list of Korean-ready devices available on AT&T:

Motorola Backflip (no Korean SMS, yet)
LG Olivine CF360
LG Xenon GR500
LG Shine II GD710
All BlackBerry devices
All iPhone models

As far as SMS, we've tested that they all work within the AT&T network and to users on all the carriers in South Korea.