One more person waiting for the answer to this mystery! i have a 3ware 9650SE with 7 WD5000YS drives attached. Firmware is, and has been since i bought the drives, 09.02E09.
I get random disappearing drives from the RAID. Interestingly it has seemed to subside over the past 6 months to the point that i barely get drop outs anymore and usually a power cycle fixes it. Still, my only solution was to have a Hot Spare constantly available to the 9650SE so when a WD dropped off the RAID it would still be redundant. When the WD randomly re-joined the RAID i would then make it the new Hot Spare.
In regards to the read performance, one word, WOW. So slow. 60mb/s or so...MAYBE. And it's very jumpy, jerky to boot. I spent so much money on this setup only to get this. Oh well.
Here's to hoping 3ware or WD come up with a fix! (meaning i'm hoping 3ware does because WD doesn't seem to care much).