NextWorth, one of a number of sites that allow you to easily trade-in and sell your electronics, has put together a chart showing how the average eBay selling price for the original iPad 1 declined as the iPad 2 launch neared.
You can see that in the two to three weeks prior to the iPad 2 launch event (the announcement, not even the actual retail launch date), iPad prices declined steeply. Since the iPad 3 launch event has been announced for March 7, that period has already passed and prices should already fallen precipitously.
In addition, NextWorth notes that wi-fi only iPad values hold up somewhat better than 3G-enabled iPads. The company believes the reason for that is that since there is no data plan to be paid for, consumers are willing to pay more for the tablet itself.
Notably, NextWorth locks in values for 21 days, so if a consumer gets a quote today, that value will hold true, as long as the device is dropped off at UPS by the 21st day (and NextWorth provides a free shipping label, to boot).
The time to trade-in, therefore, is now, the site says. NextWorth says it has seen an 814 percent increase in iPad 2s being sold by their owners "from January to February already."