Professionalism in dress is a slightly weird thing in library land, since as a professional culture we are unconcerned with clothes and this usually means dress guidelines are something along the lines of "erm, yeah, wear clothes".* This tendency is mirrored in academic culture, and therefore even stronger in academic libraries. For instance, I have often heard the advice that when working in a service profession one should try to dress about one step nicer than the average patron. Our average patron is a science grad student, so as long as my nerdy and/or swag t-shirt isn't stained or a legitimate bio-hazard....
If we have a dress code I have never heard of it. On the other hand, this attitude means people find it odd, even a little suspicious, when you dress at all fancily. I have never been told I was dressed too casually (even on that day that I actually wore my one pair of jeans and a t-shirt, in fact I believe I was complemented that day, but it was a nerdy t-shirt), but have gotten a comment or two suggesting I might be a bit over-dressed. I personally as both disinclined and don't feel comfortable dressing as casually as I could, not just get away, but be considered perfectly well-dressed, with.
Below are two work outfits. The first is in fact the upper limit of formality that I can get away with (and I think might have elicited comment had the Dean of Libraries not been visiting that day). I actually myself think of this one as pretty office-appropriate and would consider wearing it (with different hair, foot and leg-wear, obviously) to an interview or something.
Blouse: Lane Bryant outlet
Skirt, Socks: Target
This one is obviously much less formal and, while eccentric, considered totally acceptable by my colleagues and boss.
Tights, Hat: Torrid (best opaque tights ever)
Shirt: Think Geek
Skirt: Rose Mortem
Same shoes as above
Hair ribbons (for both, son't know why I have been in such a braided pigtail mood lately): 1$ ribbon bin in Michael's
*Except for front of the house public library work, which has most of the usual constraints of dealing with the public.