As I said in my comment, float: left
forces display: block
.
Here’s the relevant information in the spec:
http://www.w3.org/TR/CSS21/visuren.html#propdef-float
The element generates a block box that
is floated to the left.
And then:
http://www.w3.org/TR/CSS21/visuren.html#dis-pos-flo
Otherwise, if ‘float’ has a value
other than ‘none’, the box is floated
and ‘display’ is set according to the
table below.
To summarize said table: float
= display: block
.