The Buyer's Guide on this site will give you a rough idea based on past data, but no one is completely certain. And any answers you get regarding features will be nothing but pure, unadulterated speculation, so take all of it with a grain of salt.
Normally the end of August or in September. This year it could also be end October with Leopard and/or new MacPro's. But keep in mind Apple is planning to make all there LCD display with LED backlighting. For now we haven't heard much rumors about that nor that any manyfacturers (and then I mean LG) has brought them out for a reasonible price. I think we are waiting on that. Also any design change in the MacPro's will reflect a design change in the ACD's. Furthermore there are probably some upgrades; 23" will make room for a 24", 160 ppi instead of the 96.
LED backlight, maybe. Higher contrast ratio, brighter panel, cut 23", replace with 24", maybe a size above 30" (some rumours indicate 40"). Further price drop on smaller displays. These are all possible.
I've also been hunting for any updates on Cinema Displays and they are hard to come by... Some of what folks are waiting for, in no particular order are: built-in iSight, HDMI, LED backlit displays (promised by Steve himself in April).
I have heard - on EXCELLENT authority 😉 - that the MBP 17" LED displays are about to be released. About as in 2-3 weeks. So, if they now have 15" and 17", how much further behind could 20, 23 and 30" models be?
Does seem like they could/should (wishful thinking ahead) put out new models to show off Leopard in October... 🙄