The reason this isn't high priority for us (or a majority of our customers) is that users don't read the changelog. We did a test several years ago and found the average time "reading" the changes was about 1 second. In other words, on average the user skims the info and updates.
That being said, we're going to be adding the ability for HTML changelogs in wyBuild 3.0+ (more than 8 months from now -- Q3 or Q4 of 2011).
It wouldn't be even hard to implement what do you think?
To do it right? It would take a bit of time.