I had this happen when I had a set of <p> tags nested in a <div> that contained imgs. My alts & linked images didn't work and you couldn't even right click the image to view its properties. To fix my problem I changed the set of <p> tags to <div> tags.
The IE 6 issue was nothing...before I fixed the problem the whole div was wacked in Opera and was completely invisible in Mozilla. Changing the one set of tags got all the browsers working and looking the same.
AFAIK, only Windows browsers use the alt text in this way (perhaps only MSIE). The purpose of the alt text is to be displayed as an alt-ernative when the image is not (eg. in text browsers) not as a 'tool-tip'.