Holly Buchanan at Marketing to Women Online has an interesting posting about "underdressing" for public speaking events. In the post, she discusses Guy Kawasaki's premise that you should overdress, i.e. never dress beneath the level of the audience that you're presenting too. The discussion centers around the "why" this is important and the different perspective she has vs that presented by Guy.
Personally, I think the key is to dress comfortably and appropriately for the audience that you're communicating too. This could mean a parity dress code with those in attendance or it could mean a higher level of dress or "overdressing" using Holly and Guy's terminology. I would agree that "underdressing" isn't appropriate, because it does make you look less than professional, but think that there is room for a parity dress code that still makes you look professional and appropriate for the audience.
As the Web Chef, I've gone out to speak to audiences wearing a chef's toque and a traditional chef's jacket. This is part of my brand and "shtick" as a presenter. On other occasions, I've worn a tie with cooking instruments, food or chef's toques to reinforce my brand.
Some other dressing tips
Laura Bergells another presentation expert, pointed out the importance for women of avoiding the jangly bracelets or earrings when giving an online presentation, since the noise can be picked up and distract in the presentation.
A presenter I heard at a National Speaker's Association meeting a few years ago suggested that you not carry coins in your pocket and that you always have some safety pins available when traveling for that potential wardrobe malfunction. Unfortunately I can't remember his name.
Any comments on what is an appropriate dress code and as Holly asks, does this vary by whether you are a male or a female presenter?