When I was looking for a bike for my 12 year-old, I found that the only time you could really bargain is when the manufacturer is offering dealer incentives. Now, the dealer won't always volunteer this information to you, but if you shop around and see another dealer offering the bike for several hundred less, you can figure there may be a rebate program in effect on that model. Bear in mind the programs change daily and the dealers really don't know when and if promotions will expire, change, or remain in effect.
Other than that, it's supply and demand. If the dealer won't take your money, then it's worth more than that to him. I will say I've always found motorcycle dealers to be a stubborn lot, I just recently saw an '03 leftover on the floor offered for just a few hundred less than an '06.