2008/2009 bikes took a double in value- 2010 was much nicer, and leftover discounting (rebates) pulled value down.
When you trade in a bike, dealer makes profit on both ends, from sale of new bike and the trade you give him. That is how it is with cars too.
As for leftovers, you have be careful it is a good deal. For example, as soon as that leftover '08 hits the street, it becomes a three year old bike. While that might not
bother you now, it will if a month later, some soccer mom hits you in her SUV and totals the bike.
Most vehicles (irregardless of type or make) follow the same basic depreciation curve. First year is the biggest, then a smaller amount each succeeding year. Keep it long enough and depreciation can stop and it'll be worth as much as other similar bikes a little newer. For example, check out the prices of Goldwings that are 8 years old and ten years old- they'll probably be similar. If you are worried about value, the best bet is to buy a slightly used '10. That lets someone else pay the premium for new bike smell.
As for HD values- I think it a myth.HD is most popular with working class folks and they are the most likely to be unemployed. In my area, HD is very popular with people in the construction trade and the ads are clogged with blinged out HD bikes for sale. Problem is- how many people are out there that want to buy a blinged out used HD? The glut depressed prices. Some owners are stubborn and they'll keep them up for sale for months (like my wife's hair dresser). Personally, I check every now and then, wouldn't mind picking up a Sportster 1200 super cheap from a desperate owner who has the wolves barking at the front door.