My previous job as a programmer/sysadmin for an ISP was somewhat more casual than my current job. It wasn't unknown for me to show up in shorts and t-shirt; usually removing my shoes quite quickly.

At my current job - where I'm a sysadmin - when I'm only in there for a part day (which is currently whenever I'm there - I'm part time) I can wear smartish casual which equates to cargo pants and whatever t-shirt I'm wearing. During the holidays when I'm there fulltime I'm expected to wear a shirt and tie. However they've seen the light and it looks like we're either going to move to a smart casual regime or a corporate uniform (which at least will hopefully be more comfortable than a shirt and tie).

Oh for the days where I could were shorts and t-shirts and when the customers came in they didn't think anything of it...