I do not know about the Trek 1500, but I was at my LBS a few days ago looking at the Specialized Roubaix Elite and the Trek Pilot 5.0. While I was test riding the bikes, the fellow who was helping me called Trek to determine the availability of the 2006 models, when the 2007 models would be available, and what the changes would be. He already had the 2007 Specialized catalogue, but he called Specialized to see if the 2006 models were still available and when the 2007 models would be shipped.
As far as we could determine, the only change in the Trek Pilot 5.0 for 2007 is the color. There are no 2006 available from Trek, and the 2007 will not ship until late August or early September. The MSRP for the 2007 should be the same as the 2006.
The 2007 Specialized Roubaix Elite will have the 105 ten speed rear derailleur and ten speed casette instead of the Ultegra nine speed, it will have Mavic wheels (I do not remember the model) instead of Alex, and it will have a new Specialize Roubaix tire instead of the Specialized All Condition Pro. The MSRP will be about $50 more than the 2007. The 2007 is already shipping and my LBS will have them by next week. My LBS still had one 2006 Roubaix Elite in my size, but they told me that the Mavic wheels and the ten speed 105 insted of nine speed Ultegra were well worth the difference in the price. Because they had only one 2006 model left, they would only discount it by $50.
The 1500 could be a different story, but it looks like the major changes in the Pilot line came between 2005 and 2006, and that there will be few, if any, changes between 2006 and 2007.