Using rst2html.py (Docutils 0.5 [snapshot 2007-06-07, r5227]) a figure element in the input rst file is translated to a <div> in the output html. This inhibites text from flowing around the figure by default (at least when viewer with firefox 2). This can be fixed in the stylesheet by adding e.g.
margin-left: 2em ;
float: right // <--- added
but the value for the float field obviously depends on the alignment of the figure (so a different CSS class would need to be added for each alignment)