I was reading this press release from Chi Mei Optoelectronics, which provides LCD panels for Apple's notebooks. The press release describes some panels that are on display at Display Taiwan 2007, a trade convention going on right now. Among the panels is a 22" size that "use mercury-free LED backlight modules for enhanced environmental protection and high color saturation and is expected to become the display of choice for home office and entertainment purposes." I think it's reasonable to infer that this panel will be the basis for next mid-model Cinema Display. As with the Macbook Pro, not all the models will be converted to LED panels at once but it appears the mid-model range - 22", 23", 24" - has reached a point where an LED version is being manufactured economically. Here's the link to full press release:
http://www.cmo.com.tw/cmo/english/about/shownews1.jsp?flag=20070612154119
http://www.cmo.com.tw/cmo/english/about/shownews1.jsp?flag=20070612154119