Obviously, there will be a slight learning curve for your designers. But with that said, I have worked quite often with designers (none of whom used Dreamweaver, btw, so that may be part of the problem) on asp.net sites. Usually, they will create the HTML exactly how they want it on the server like a static HTML page, then I will go in and replace form fields manually with asp.net controls.
On an aside, I have found that I have the best chance of matching the design using controls that spit out the least HTML, such as Repeaters instead of DataGrids.
Once the site is up on the server and programmed, they can go back in an tweak things if need be.
Also, just like we have to adapt to them a bit (making our server controls spit out html how they like it) they also have to adapt to us a bit and not rely as heavily on id attributes in their stylesheets as some items id attributes will be controlled by hte .net runtime since they are controls.
MOre often than not, a designer new to asp.net will feel very threatened by this new way of doing things, specially with user controls instad of include files, but its really not that different than classic asp/php development is.