Code:
CSS
div#content div.section div.region-content {
float: right;
position: relative;
}
Is this DRUPAL template CSS or your own? That seems like a helluva lot of specificity for a basic block. Maybe just put an ID on the block. That should be enough.
IE is funny with floats. Is there a position: relative in the parent container properties? Or on any grand parent above your float in the cascade? Seems to me that "haslayout" needs to be on in order for float: ____; to work properly. Another thing may also be the order. Whenever I float something right, I do it before any other floats (such as a float: left; container).
The solution may be as simple as changing it to a left float, and just making sure it will always fall in the same general position.
Another question: How many nested floats are there, if any? Sometimes this is buggy. I know this is probably not much help, just spouting all this stuff off, but it gets the juices flowing and gives you more terms to search for. Have you checked for this issue on quirksmode?