One thing I like to do when those things happen is to just put different colored borders around everything and see where things are initially...
<div style="width:900px;font-size:14px;font-weight:bold;font-family:arial,helvetica,swiss,sans-serif;color:#585858;text-align:center;border:1px solid blue;">
<span style="float:left;padding: 0 2px 0 2px;background-color:#dcdcdc;cursor:pointer;border: 1px solid red;">Some Text</span>
Here I added a blue border around your div, and a red one around the span. Makes it easier to see what changes whenever you change your code. and when your done, just turn them off.