One of the issues people are having with tabs on top in Safari 4 is that it's hard to distinguish which tab is currently frontmost. I found this an issue even with the previous location of the tab bar in Safari 3. The problem, for me at least, is that the Unified interface theme doesn't provide sufficient distinction between the tab which is frontmost and those which are in the background. I don't have this problem with the interface in other areas, like the Finder, and I suspect it's because windows don't get grouped as closely as tabs do.

My solution is simple and only takes me a few minutes:

1. Quit Safari then navigate to it in the Finder.
2. Control-click on Safari and select Show Package Contents from the pop-up menu. Navigate into Contents » Resources.
3. Make a copy of the following four files somewhere outside of the Safari package to serve as a backup: AW_ActiveTabCenterFill.png, AW_ActiveTabLeftCap.png, AW_ActiveTabLeftCapFirstTab.png, AW_ActiveTabRightCap.png.
4. Open the original files (within the Safari package) in your image editor of choice.
5. I added a two-pixel wide red border to each image as indicated here:
   • AW_ActiveTabCenterFill.png: Add a border only at the top.
   • AW_ActiveTabLeftCap.png: Add a border on the left side of the image. (I didn't add the border to the curved portion at the bottom left.)
   • AW_ActiveTabLeftCapFirstTab.png: Add a border only to the left side of the image.
   • AW_ActiveTabRightCap.png: Add a border only to the right side of the image. (I didn't add the border to the curved portion at the bottom right.)
   I used a red color because I find it's easy to locate when I look at the tab bar, but it's not visually distracting the rest of the time. Others may, of course, prefer a different color.

For users of Safari prior to version 4, the same effect can be achieved by modifying these files in a similar way, though I find a one-pixel wide border is sufficient: TabBevel_Caps.tif (left, right and bottom edges) and TabBevel_Middle.tif (bottom edge only).

[robg adds: I tried this, and it worked very nicely, though I had to use PNG-24 to insure a transparent background. As an experiment, I took the editing even further, and replaced the entire gray gradient with something more colorful: With a colored tab, I find the top tab bar location doesn't bother me nearly as much as it did when all the tabs were a near-uniform shade of gray. Because these files are under Apple's copyright, we cannot distribute modified versions, but only instructions on which files to modify.]