Yes, this is a pretty serious problem. It isn't just a problem with dial-up. I have a 384K DSL connection and see it all the time. It can also happen when loading from slow servers regardless of your Internet connection. I see it on pages with lots of image thumbnails coming from slow servers. I also see it when I load several pages at once (such as command-clicking on several links to open pages or full size images in separate tabs).
Under Safari 2.x, pages would load as best they could and either finish loading, or the images that failed to complete would still be partially visible. Under Safari 3.x, any image that doesn't load by a certain "time out" changes to a ? icon. Plus, in Safari 3.x, images that are partially loaded are displayed while trying to load, but then when it times out, the image is cleared and replaced with a ? icon. This is frustrating because often the image has loaded, but it's waiting for some "completion" flag to say the image was loaded, or maybe it's still loading a better quality version of the image, but the lower quality image is completely visible - until Safari 3.x erases it and replaces it with a ? icon.
Again, this isn't just a dial-up problem. It's a problem with loading any web page that takes too long - no matter what your connection. For example, this web site is a perfect example:
http://www.pixolator.com/zbc/featured2col.php
Another example are forum pages with lots of avatars and images in the messages. Because the images come from so many different sites, fast and slow, I often see only half the images. This link is a good example:
http://www.stickboyracing.com/stickforum/viewtopic.php?t=154&postdays=0&postorde r=asc&start=0
This is a pretty inexcusable error in Safari, and it has existed for over a year (ever since Safari 3.x was first released I believe). It has made Safari unusable for reading many forum pages, and accessing any web page with a large number of images or thumbnails.
The only solutions I've found are to use a different browser, or click on a link a couple times to reload the page
without doing the equivalent of a "refresh" on the page so it loads images out of the cache the 2nd time I click on the link, and reloads those images that didn't work the first time.