Normally, alt tags are not visible. They become visible if you have images turned off or if images do not load properly. They are also there for accessibility purposes so that you can have the computer read the page to you (and for that reason alt tags are required for all images if you are to meet the W3C html/xhtml standards).
When I turn images off, I do see a difference in the display of alt tags between IE and Safari. IE displays all of the alt tags but Safari does not. If an image is also a link, then Safari does not show the alt text, but the area is still an active link. Personally I think the alt tag should be displayed for all images as in IE, so maybe a bug report using "Report Bugs to Apple" under the Safari menu is in order. I also notice that on the couple pages I've used to look into this issue that in most cases Safari has a box where images would appear, but in some cases there is no indication that an image is supposed to be present. IE had a box for all images.
Another issue is that some browsers, against the established W3C standards, have created or used to create tooltips from the alt tags, which is not their purpose. There is a separate title attribute for tooltips. However, neither IE or Safari (or any of the current OS X browsers I can think of) incorrectly display the alt tag as a tooltip.
