''A MailingList, on the usual venues, discussing the effect of agility on big usability design up-front.''

I break UI design into two parts - visual (or graphic) design and interaction design. If you are creating UI guidelines, the visual design and interaction design should be separate, with the interaction design process starting first, IMO. 
 
When I'm figuring out the interaction guidelines, I'm working out the button placement (consistency is super-important here) and names (such as when to use "Cancel" or "Close"), pagination standards, how the page will be dynamic, when and where tabs will be necessary, local and global navigation location, widget use... and so on and so forth. I then begin a wireframe document where the calling out the pages' behavior and general layout is spec-ed out. Sometimes an HTML wireframe prototype is made at this point (if designing for a web page or web application).
 
Once the ID guidelines are drafted, the visual design guideline process can begin. Color, typography, font size, branding, icon design and motion graphics (and sometimes sound) are all decided. The visual design guidelines dictate how to best present the pages' behavior. Where interaction design is the skeleton (ala - wireframe), the visual design is the skin. Many times the visual designer is responsible for a HTML prototype using the visual guidelines
 
Here's my reason as to why UI design should be separated like this - an interaction design review can be completely sidetracked with issues like, "I don't like that hue... make it bluer", or "why did you use that font?" - things that have little to do with the pages' behavior. If the interaction guidelines are figured out first, the visual designer has a foundation to build on and time to focus on creating the pages' final layout and the visual design styleguide. Even if the visual design and interaction design guidelines are created by one UI designer, it's good to separate the guidelines into the two groups. Once the visual and interaction guidelines are finalized, they can be incorporated into a single UI design guideline document and passed on to the developers and content writers. 
 
What does everyone think? This is only how I (ideally) create UI and I'd love to hear how others do this. I'm new to the agile method of development, so perhaps someone could give some insight as to how this process might change as a result. 
 
Kevin Doyle
IA and UI Design Consultant
CC Pace, Inc.