There isn't going to be a refresh soon. The 24" LED is already using state-of-the-art technology and the highest grade components. H-IPS, LED. This stuff isn't going to be outdated for at least another couple of years.
The original 20, 23 and 30" displays launched in 2004, got a slight brightness/response rate boost in 2006, and then the 20 and 23 were replaced in 2008. The 30" is still going! It'll be relatively long time until we get an LED display refresh. I say: Buy now! It's on Apple's refurb store for $699.