When you're designing websites, do you consider yourself responsible for implementing search optimization elements into your design?
Yes but only to a point. Frequently the designers hands are a bit tied when it comes to SEO. Its usual practice for a design to be handed off to a front-end dev who may or may not have any experience with SEO or simply has their own coding style/framework that would require a significant amount of extra effort to include optimizations. And lets face it, very rarely will a project's requirements doc specify search optimization. In those cases where its not a specified goal, many people will simply leave out things that will help in ranking because it wasn't required.
In addition, even the designs/devs that implement correct semantic search-able structure can easily be defeated by a client who's CMS allows them to change headlines, ignore alt tags and so on.
All this to say that while yes this is something I consider (and others should as well) its frequently out of my hands long before a site actually launches.
I can definitely confirm through many tests that the faster loading css based sites rank much better than their slower table based cousins.
I'm not convinced that speed or coding structure has much to do with ranking. Its simply that when using semantic CSS/HTML you are taking a step to separate the visual look from the content. This step requires the code to contain a certain hierarchy and structure that is inherently easier for spiders to follow and index. This structure could be replicated in a tables based layout by using decent body copy that contains keywords, proper headers for headline content and so forth. The issue is that table layouts are typically only used by newbie designers who don't know the implications of that structure let alone how to make it search capable.
I've worked with a lot of designers, and it sort of seems "luck of the draw" when I get a designer who considers the online marketing direction of the site in terms of SEO during the design phase.
The bar for entry to the web industry is extremely low and the entire industry is therefore flooded with people. However, when there are so many "designers/devs" available there's no way to expect the same type of quality across the board. This is the reason why you should carefully choose who you work with based on their experience and expertise. I'm always amazed by the people who hear my rate and then say they know so and so who will do the entire project for $50 (insert some ridiculously low number here). These people don't understand that someone who understands how visual design, front-end dev best practices, current technology all fit under the umbrella of marketing strategy is a highly skilled expert. However, these people also rarely understand (or care) that the designer/developer should be able to improve their rankings simply by adhering to a best practice based process.
If at first you don't succeed F1... If that doesn't work try Google!
//// Designer, Developer & Teacher - Interactive, Motion and 3D \\\\
Portfolio at WhenImNotSleeping.com