Okay, I feel like I should take a swing or two at this as well.
In the following two cases, there isn't any special font being used. I'm just using inkscape to trace out the characters that I want show...Of these two logos that I sent, my preference is for the second.
My personal opinion is that you shouldn't try and get too hung up about the acronym that you are trying to capture and represent... (I say this even though I did my best to incorporate the letters... blah). I think sometimes that capturing the concept is more important than capturing the acronym.
The logo is incredibly simple and doesn't try and spell out USB.. however, it does capture the essence of the interface and what it seeks to accomplish and I think that is what makes it memorable.
That is really all I wanted to drop by and say.
-Mike