Ultimately it depends on the relationship you have with your dealer. I've bought 5 BMWs in the last 15 years from the same salesman (and now he's the sales mgr) at the same dealership. I know how they work, and they know I'm ready to buy when I call them up.

There is a promotion right now that offers a free premium package. On a 4.8, that's $1850 off the price right away. Apparently dealers can opt out of this, but I'm not sure why. It probably affects part of their holdback or something similar.

I was personally able to quickly get to the price of $1000 under original invoice, but because of the premium promotion, it was $600 or so above the real invoice, so they were able to make some money on the deal. Total time involved: 2 emails on a Saturday and a 1 hour visit on Sunday to finalize the deal and shake on it. It also helped that this was at the end of the model year and end of the month.

At the same time, I was able to do this because we were talking about a car that they had no money in, so there wasn't so much pressure to make a larger profit. If I was offering on one of their limited allocation, already on the lot and incurring interest, etc, I don't think I would have been able to get anywhere near what I got.

There are lots of posts on here about end-of-08 pricing. I suggest you spend a bunch of time reading posts and the pricing docs in the stickies.