Congrats on the new job! You are absolutely correct about the business casual/jeans aspect of the dot.com industry (been there, done that, bought the tee shirt). I find that many women who work in a casual office make the mistake of being too laid-back in their attire. The key is in finding a good balance between casual and professional without, as you mentioned, looking like you are trying to be a college student. The good thing is that there are so many alternatives these days to the basic jeans and tee shirt.
For footwear, since you are petite, you can go with a platform or wedge heel for added height as well as added style. I love platforms because they are easier on the arches of your feet and more comfortable. Same with wedges. I also find that a more substantial heel is great for ease in movement.
Finish with workable accessories. Bracelets and bangles tend to get in the way when you are working at a computer, so go with statement necklaces and fun earrings. And don't forget a great handbag for toting around all your goodies!