In IE the report viewer control cuts off the bottom of the report, including the horizontal scroll bar, and if you have parameters at the top possibly some of your data too. To make it show the whole report and the horizontal scroll bar add style="margin-bottom:50px;" to the report viewer control.
Update 12/03/2010: Actually there is a bug with the report viewer control in IE8 that sometimes prevents horizontal scrolling going right accross the report. There are some suggestions to add display:table to the report viewer which does make the horizontal scrolling work but can break the height. The only real fix is to remove the doctype to put the brwser in quirks mode, but is not really desirable, especially if you have the doctype in a master page and want it on your other pages. So in the end I found the best way to get it to display right accross all browsers is to put the report viewer control in a seperate file with no doctype and include that in an iframe, then you can forget about all the margin and display styles.
Update 12/03/2010: Actually there is a bug with the report viewer control in IE8 that sometimes prevents horizontal scrolling going right accross the report. There are some suggestions to add display:table to the report viewer which does make the horizontal scrolling work but can break the height. The only real fix is to remove the doctype to put the brwser in quirks mode, but is not really desirable, especially if you have the doctype in a master page and want it on your other pages. So in the end I found the best way to get it to display right accross all browsers is to put the report viewer control in a seperate file with no doctype and include that in an iframe, then you can forget about all the margin and display styles.