I don't know about Trek, but 2009 prices could increase in some brands close to 20% from what I've heard.

1) If you can get the right fit on the Madone and already have the Reno set up more aggressively than you did to start, I think you can probably adapt to the Madone just fine. It could help you in endurance rides, too. For example, the philosophy behind the Specialized Roubaix is to make a performance bike that helps reduce rider fatigue. So if you can get enough comfort with the Madone plus the Madone's increased performance, you may save watts riding that bike plus get a smooth ride, albeit with a more aggressive body position. If your back can take it, you may find that the better bike will be just as easy to ride in endurance competition as it is in a regular road race.

2 and 3) You WILL notice the 6 lb difference. You will notice the better, snappier ride of the Madone carbon frame. It will corner better, climb better, and accelerate better. As for upgrading components in general, I haven't heard any of my friends say they like Ultegra or Dura-Ace as much as their older Tiagra or 105 groups. The lower groups may be perfectly adequate, but there is a reason the other stuff is better. Crisper shifting. Less tuning necessary. Weight savings. More ergonomic feel or ease of use. How much you appreciate an upgrade is personal to you. I believe you will like better stuff, but that you will notice the change in the frame and overall bike weight a lot more.

4) Ultegra-SL has pretty much the same functionality as DA, but is slightly heavier (but slightly lighter than regular Ultegra). At least I'm assuming you mean 2008 DA. 2009 DA is another animal, and is supposedly "the best," but you will need to buy 2009 DA chains and cassettes for best performance (though I hear that the chain wrap issue may just be Shimano blowing smoke), which can add up for sure. So, for 2008 DA vs U-SL, it's kind of a toss-up. I'd see what kind of deals you could get for either build, or do a mix of the two. Like, go with SL brakes but DA shifters. Between the 2, there's not as big of a jump as between regular Ultegra and DA.