FF and Chrome work fine with the hack above.
IE still refuses to display the pictures.
I believe this has to do with some wicked "security" feature of IE which blocks the retrieval of something which does not look like a picture.jpg but is generated on the fly.
I did a test to add a single line in a testme.html file as provided by google during the "publish this graph" operation, namely:
<img src="http://spreadsheets.google.com/oimg?key=0AtH_72LyVY3XdFd1TU04OHludWtRMElRR2NCc0V2bkE&oid=1&v=1270132133024" />
I get a box with a red cross, which means that the picture is "broken" according to IE. FF:Chrome like the file and display the chart.
If someone would be kind enough to do the test (create an html file with the chart above, or another one generated the same way) and open it in IE I would appreciate the feedback -- this would tell me if my IE is broken or if this is a global problem.