I'd love to have this, too!
However, there is no way to do it with Safari as is.
Luckily, though, Safari 5 is
open to extensions. This means that we may soon see a third party extension that lets us do this.
In the mean time, here's some advice for saving space:
First of all, the obvious:
1. Short Names: Shorten the names of your bookmarks. If you can use just one letter to remind you of what it's for, great. That will save space. Also, if you hover over a bookmark, you'll soon see a tooltip telling you exactly where it goes.
How: Right click on a bookmark and choose Edit Name. Type a new name and click OK.
2. Folders: Put things that you use less in folders. This way, you can fit as many things as you want into one Bookmarks Bar item. The drawback is that it takes a little longer to click them.
How: Click the book icon in the left of your Bookmarks Bar. Click Bookmarks Bar from the list on the left. Click the + button below your list of bookmarks. Name the folder, and drag the bookmarks you want into it.
Next, the less obvious:
3. Keyboard Shortcuts:All bookmarks appear in the Bookmarks menu (in the Menu Bar at the very top of the screen). System Preferences lets you add keyboard shortcuts to any menu item. So, this means that you can take bookmarks that are either in your Bookmarks Bar, but not visible; or in your Bookmarks Menu, and activate them with a hotkey instead of the mouse.
How: First, make a bookmark for the site you want. Either put it in your Bookmarks Bar, and drag it off to the right until it's in the little >> menu; or put it in your Bookmarks Menu. Be sure to give it a unique name, i.e. one that isn't used by any other bookmark or menu item. Then, open System Preferences. (The following directions are tested in Snow Leopard, but shouldn't be too different for Leopard.) Go to the Keyboard Shortcuts tab of the Keyboard preferences pane. Click Application Shortcuts in the list on the left. Click the + button below the list of shortcuts. Set the Application: menu to Safari. For Menu Title:, enter the name of the bookmark you just created. In the Keyboard Shortcut: box, press the shortcut you want to assign to the bookmark. Make sure that this shortcut isn't already used for something else (I recommend the Control key; it's pretty much unused). Quit and reopen Safari. You can now press the shortcut you chose instead of clicking the bookmark.
Note: you can do this for existing bookmarks, just use their name in System Preferences.
Sorry I can't provide a direct solution, but these may help.
-Nate
