Good to hear that someone else is working on these. We had a couple of WC sacs late in 2006. I just had a female that molted in late October, and was bred to a WC MM in early November, drop a sac. It looks great, but we'll just have to wait and see.
I can definitely say that all of the confirmed gravid females I've seen are incredible fat. The opisthosoma swells, and becomes very black and smooth, with stretch marks being present all around it. My females that have molted never became this fat.
Congrats on the sac, hope all goes well. Both males that I have had were captive born ones that I reaised up, so it took a little longer to get them ready. I have a smaller female that I may try with this male but I will see. My friend has a female that is not that much smaller than my big girl and she will hopefully get bred too.