Very blinkered attitudes here
What's very evident here is that are some very blinkered attitudes here in these comments and while that's not be unexpected given the techie readership, it is a little disappointing when smart people don't understand the value of appearance.
Firstly, everything we wear is effectively a uniform - it doesn't matter if it's a suit, grey hooded tracks and a flourescent yellow fly attractor, jeans and t-shirt, summer dress, cocktail dress or skirt and blouse. It's all a uniform of sorts and the clue is in the word "uniform", as in a common appearance - to whatever paramaters you assign "common".
First impressions do matter and we are perceived by what we wear and our appearance. If you dress like a builder then you'll be treated like one because that's what you look like and you'll find it easier to relate to other builders as a result; we only have limited time in our lives and that doesn't allow for asking every person we see that's dressed as a builder whether or not they are a builder or are in fact a security guard. And so on... security guards dress in a distinct manner so they can be recognised as such and of course this is used in reverse where sometimes they pointedly don't dress as one so they aren't recognised.
So rather than hissing and spitting about how bad it is that some companies have dress codes (aka uniforms), start with the understanding that how you dress really does affect how others see you. This is still the case even if you claim not to care how anybody else dresses, although if in your next statement you claim that "everybody in a suit is evil" you evidently have a false = true problem. How we dress is a tool we can use and it's one I learnt that lesson a long time ago and even directly experimented with it at trade shows where my business partner and I took it in turns to wear a suit or more casual clothes and the change in how we were treated even while standing next to each other was dramatic.
Not that HP's sudden insistence on a stronger dress code will make a lot of positve difference to such a large and laughably inefficient company...