I am a very strong believer that the designers should provide the implementation of their art which might include the following assets: HTML & CSS, Flex/Flash, mobile layouts, stretchable images, etc. If the designer can come up only with wireframes, or with the static Photoshop images, then they are not doing much favor to the end product.
It is sometimes worthwhile to hire a design firm to provide an idea and creative direction, but each product development team will need an artist sitting together with developers and working together. The practice of throwing the design "over the fence" to technology department, as it is common in many companies will result in a terrible product. If the company considers the mobile product as a long term strategy then the artists should become the core part of the team.