It is very unlikely that Apple would release new hardware to coincide with Leopard, though a speedbump is possible. Apple will ship new hardware after Intel ship Penryn. Now is a good time to buy if want something this year.
A step down from what? I just took some standard display resolutions and provided the sizes at 125dpi. Realistically, if Apple were to offer a higher resolution Cinema Display at 1920x1200, I think it would make more sense to use the same 133dpi display now optional for the 17" MBP.
Of course, it's possible that Apple might ship Cinema Displays with non-standard resolutions.
If Apple were to ship a new line of Cinema Displays with 125dpi, they might be:
30.4" WQSXGA 3200x2048
24" WQXGA 2560x1600
18" WUXGA 1920x1200 (Apple already offers this resolution at 133dpi on the 17" MBP)