So I forgot to include in my post earlier in the week about my first toon hitting 90 - my total played time ended up being 28 hrs to 90. That compares really favorably to my first character that I leveled to 85 in Cataclysm, which took 35 hours for the first character out on launch day. That's not exactly a speed leveling time as I was enjoying the scenery, generally reading the quest text and also leveled both of my professions to 600 and got my farm started.
In Cataclysm, I saw approximately a 40% improvement in leveling time from my first character to my most efficient and with all the remaining characters after launch day / read the quest text character being within about 10% of each other in total leveling time.
If that carries through to Cataclysm, then I expect the remaining toons to take somewhere around 18 hours each. We'll see how that pans out though.
I've started on the Death Knight next, although I'm still spending the bulk of my time on dailies and heroics on my Warlock, trying to get him geared up for LFR when it releases in another week. The biggest difficulty so far seems to be Trinkets.