Could this cause penalty?

[bcc1234]

Instead of using pixel images for lines, I use divs as it prevents from having lots of requests for small files.
It gives pages a much quicker perceived loading time.

So instead of

<img src="blue_pixel.gif" width="200" height="1">

I have

<div style="background-color: rgb(0, 0, 255); width: 200px; height: 1px; font-size: 1px;">&nbsp;</div>

Could Yahoo's spiders interpret that as hidden text?

[WebGuerrilla]

No.

[flex55]

No, from me too.

[prairie]

I'd say no -- but non-essential html may dampen rankings.