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)
Until most applications have adapted to Resolution Independence, we won't see Cinema Displays with the 163dpi of the iPhone. However, with Resolution Independence about to ship, it's time to start offering 125dpi Cinema Displays. That would correspond to a 24" monitor with the 2560x1600 resolution of the current 30" monitor.
What could Apple release in October? What technology will be available? Nothing that would justify a new MBP. While a speedbump can't definitively be ruled out, I don't expect one. Either buy today or wait for Penryn.
Why on earth do we still have bit-mapped UI elements? OS X has had Display Postscript since it was NextStep 0.6 in 1989. Is Apple still trying to maintain compatibility with software written for OS 9? It's time to let that go.