Ben,
take a look at http://www.7-sisters.com for an example of a site with the navigation in fixed left hand bar. The layout uses CSS extensively and thus requires CSS compliant browsers. Be aware -if you choose this path- that there are still many differences between browsers when it comes to CSS. But at least it is a lot better than frames ;)
Don't forget that OpenACS is a developers toolkit. Practically all projects replace the default design for their own. While your suggestion has merit, out of the box design is not a top priority. Having said that, the community will putting more effort into separating (Tcl) code from presentation. The next OpenACS release will see better and more extensive use of the OpenACS templating system allowing rapid design changes.
/Bart
