This is easy.. you mean like what I've done on
http://www.artificialwings.info
You need tables. First, select the bottom and top portions of the "box" thyat you want to expand ( by top and bottom I mean, the top ----- broken boundaries and the bottom one ), and then save them as say top.jpg and bottom.jpg, then with marquee tool select a rectangle portion of your picture that you want ot keep expanding asy ou fill with text.. save it as "middle.jpg" heres how you put it together:
<table width=whatever width heigh=100%.
<tr>
<td background="top.jpg">
</tr>
</td>
<tr height=100% width=the width of your middle piece>
<td background="middle.jpg">
text goes here.. the middlesection that you cut out of your contentbox thing will keep expanding as long as th text.
</tr>
</td>
<tr>
<td background="bottom.jpg">
</tr>
</td>
</table>
There you go