I’m in non external client handling IT. Everyone on my team would agree with @anomander … funeral!
As it applies to my company, in the midwest, it seems the higher up the development food chain, the further away from business casual it gets. The Software Engineer III’s and up look like they just rolled out of bed and walked into work. The IT lights (SQA’s, System Analysts, etc) seem to dress in mostly jeans and polo/ button up/ sweater. My company has a no t-shirt and tennis shoe policy, but at least 3 devs have on a t-shirt and most of my dept is in tennis shoes.