Just depends when they get more refurbs done. check back every few days. happened to me when i bought my ibook a while back. day i went to buy it was gone. checked back three days later and it was there.
I've been following the refurb store pretty closely lately and they've been replenishing stock almost daily. The unibodies don't last long, though.
You may actually have luck calling and asking about buying a refurb unibody. I was changing an order today and the guy asked me if I wanted to setup an order for something that wasn't currently listed as in-stock. Might be worth a few minutes on the phone.
Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_2 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5G77 Safari/525.20)
They tend to update early in the morning (~5-7am EST). You're most likely to grab one if you just check when you get up every morning.