Need help resolving a simple strange bug

Hello friends.
For some reason my sidebar has a strange bug (or a design error)
if the main area is long, the sidebar stays on the side.
But if the main area is short (as in short pages like contact page)
the side bar in on the side and beneath the main area too.

Please check those two examples:
in this example the side bar is on the side because the main area is long
http://www.emetaheret.org.il/2010/03/01/%D7%92%D7%A8%D7%99%D7%A0%D7%A4%D7%99%D7%A1-%D7%A9%D7%99%D7%A0%D7%95%D7%99-%D7%90%D7%A7%D7%9C%D7%99%D7%9D-%D7%94%D7%AA%D7%97%D7%9E%D7%9E%D7%95%D7%AA-%D7%92%D7%9C%D7%95%D7%91%D7%9C%D7%99%D7%AA-%D7%A0/

in this example the main area is short and the sidebar is going beneath the main area
http://www.emetaheret.org.il/%D7%99%D7%A6%D7%99%D7%A8%D7%AA-%D7%A7%D7%A9%D7%A8/

could it be a plugin thing?

I think its a css or php bug.
Please advise.

Thanks