Mine is indeed on hold now but not reflected in statement just yet. Probably the next thing I see will be the figure in my transaction statement when they are ready to ship it. Fingers crossed that's the way as I have a 19th-24th delivery date on the BTO 27"
First, let me say that I do hope that it indeed is a legit sign that your iMac is being built and will be soon shipped!
Apple usually places a hold for the full amount as a temporary authorization. The hold then rolls off in a predetermined period of time that appears to vary based upon the payment method or the institution. For example, when I ordered by rMBP, the accessories where charged immediately (separate holds), but the computer itself was put as a pending transaction and rolled off after about 5 or 7 days. My iMac hold is still in place from 11/30. If it doesn't disappear by this weekend or the date of the hold is updated, then I'll start to get excited.
On a side note, I noticed that the 21.5" BTO iMacs that were originally quoted 7-10 business days before shipment and estimated delivery of Dec. 17 - 21 have started to arrive already. One can only hope (wishful thinking, I know) that we would also see a similar pull-in on the 27" iMacs, which would place delivery at the middle / end of next week for may of us with Dec. 21 - 28 delivery estimate!