Pardon, so lets do the math.
1499 to 999 refurb 14" ibook 2 months after release of MB = > 33% drop in cost.
According to the
Mac Buyers Guide Section it has been greater than 264 days since the last revision. The average is under 200 days. Only the X-Serve has had a longer wait, at 300 days, and both are Pro machines. There are a lot of people that have been waiting for the switch. Many pro apps have gone universal binary, so there is a market filled with people who, if it is financially viable, will drop their PowerMacs for the LAG Intel machines.
An educated guess would be, from Apple.com a 17-30% drop with in the first 2 months, and from auction sites, something greater than that (over 45%)if your wise. I'd expect that to be larger if Adobe goes Universal before then. You could hone the guessing better if you do some back checking using the MBP introduction, and more refined dating.
One thing to think about is that the jump in performance between the iBooks and the MacBooks and the PowerBooks and the MB Pros was greater due to the fact it was 2 generations different, G4-Intel. Where the PMG5 - MacPro Intel may not see as much of a performance jump at first. This will make the enticement much less.