There are two ways:
The long form way (define the id as if it were a regular attribute):
.box{:id => "item_#{x}"}
produces this (x is what ever x.to_s evaluates to):
<div class="box" id="item_x">
The short form way:
.box[x]
produces the following assuming x is an instance of item:
<div class="box item" id="item_45">
See the HAML reference for more information.